Not everyone running NC on a hosted environment can convince their host provider to update MySQL to the latest version. I hereby request some sort of maintenance for NC 20. I recently upgraded from NC 20.0.13 to NC 20.0.14. I get the following message after upgrade -
I think your request is misdirected. Nextcloud is on Ver 23 now and your provider is limiting your ability to keep your software current and secure.
If your hosting company is not playing ball then move to a more flexible environment. If you move your instance to a cloud infrastructure provider such as DigitalOcean you are in control of all variables and won’t run into these issues. Shared hosting companies are great when you first start out but you quickly run into issues like these that force migration to a more “bare metal” like environment with full admin control.
I am totally with the OP. RHEL7 and its derivatives (Centos7, ClearOS7, OracleLinux7, Scientific Linux 7 and so on) are all still current products, yet no longer supported by NC as they come with Mariadb 5.5.68
… and they don’t go EoL until July 20204.
You have collision of two products:
hosting - which you are paying for
nextcloud - which you are getting for free.
In my opinion you should try to influence hosting first 
yah, I am searching, thanks for the suggestion.
I’ve been bombing them (Bluehost) for weeks. What PITA they are - “soon, very soon”.
If your NC server storage needs are relatively small you’ll find a cloud provider to be very reasonable. Before committing to a cloud provider look for introductory offers because most of them (DigitalOcean, linode, etc) will give you $50 or $100 introductory credit before signing up so you can give them a try. As an added bonus, you’ll get a dedicated IP on every machine which makes DNS work a lot easier.
Background: I tried a couple cloud providers for three company IT installations I manage. One has 250GB of data on NC and after a little over a year on AWS we moved everything to a new in house custom built bare metal Dell server because the ROI was ~ 8 months.
It looks like a fairly standard warning? Not stopping you doing anything?
I would get on to your hosting provider! It’s in their best interest to update to a supported release of MySQL to protect the security of their infrastructure
I think it’s pretty good that Nextcloud let’s you know but doesn’t stop working to be honest!
I’m in a (slightly) similar place to the OP.
I self host, and currently use an unsupported OS (Solaris), at some point in the next couple of years I will move to OmniOS or SmartOS (OpenSolaris derivatives).
There are many good reasons why I will keep using these OSes.
However, MySQL 8 has become an issue. It is of course owned by Oracle and in their usually short sighted wisdom they have dropped MySQL8 from Solaris x86 (keeping SPARC). This has a knock on effect to all the OpenSolaris derivatives.
The SFE (https://sfe.opencsw.org/) are working on the package (I am currently testing).
This has been holding me back for a while with my main NC (runs in a Solaris immutable zone, one good reason to use Solaris ), my biggest issue was the Mail app crashing upgrades for NC20 and needed to disable it.
Just saying that some of us are not always able (or willing) to run at the same pace
RHEL/CentOS 7 also came with PHP 5.4 originally. How did you manage to upgrade that? MariaDB does offer official Repos for RHEL/CentOS7… https://mariadb.com/downloads/
In this day and age, it is completely unrealistic that you can run a distro for 10 years, without upgrading any dependencies, if you want to run software like Nextcloud on it. You cannot have modern features that can compete with Google Workspace or M365 on the one hand, and use an OS with dependencies that are several years old on the otther hand. Nextcloud wasn’t a thing for nearly 2 more years when RHEL7 was released six and a half years!!! ago and they do support dependencies like MySQL/MariaDB and PHP for a reasonable amount of time imho, usually for arround 4-5 years. Within this time span it is always possible to upgrade to a newer RHEL release, if you do not want to use third party repos.
I use ClearOS7 and they have integrated the rh-php7? packages from the Redhat SCL repos. Even this causes a problem because you have to add a couple of lines to /etc/httpd/conf.d/nextcloud.conf which then breaks the NC integrity check. The good thing about the way it has been implemented is that the PHP version can selected on a per-app basis so you don’t need to check if other apps are compatible.
The trick they used for the rh-php7? packages cannot be applied to MariaDB as only one package can bind to the socket so an upgrade for NC would force the MariaDB upgrade to all other apps using MariaDB for all users. Not good.
Yeah. That’s why this classic LAMP Hosting services, like OP is using, are not feasable anymore for many things. A product like Nextcloud should run in it’s own dedicated VM imho and probably have it’s own dedicated database.
I have some sympathy for the OP, it is a pain when software companies try and enforce updates, but I don’t think this is happening here. Just ignore the message (or code it out to make it invisible - you can probably hide it with css).
Personally I am still running NC18 for my uses because it does what I want, I have some time invested in customisations to “improve” it (for me), and I can’t be arsed to waste time fixing it if it is not broke. If I was to update then apps I rely on (eg OCC) would no longer work or have their own “improvements” which I would find conflict with my needs (eg the whole OwnOffice farrago) and if my sites exists in a dusty unused corner of the interweb that’s fine. The people who use them know where they are, if they go down then they are down for a while but life goes on.
Yes I can understand it too. But sometimes the devs do not really have a choice when they wanna implement new features and at some point it also becomes unmaintainable, when you have to test your software with x ammount of PHP and MySQL versions. I think it is completley reasonable to require an upgrade of these dependencies every 4 to 5 years.
In case of the OP I would look for another hosting provider. A hosting company that does not meet these requirements in 2021 clearly has no interest in selling anything. I mean, what else do you want to host on such a server? Are WordPress or other popular PHP applications still supporting MySQL versions older than 5.7 and PHP versions older than 7.2?
EDIT:
Ok I looked it up: You can still use MySQL 5.0 with WordPress, but they also say:
Note: If you are in a legacy environment where you only have older PHP or MySQL versions, WordPress also works with PHP 5.6.20+ and MySQL 5.0+, but these versions have reached official End Of Life and as such may expose your site to security vulnerabilities.
https://wordpress.org/about/requirements/
And yes I know that RedHat backports security fixes. But I still think that when you host internet facing services that you should stick with versions, which are still officialy supported by the original developers.
Apart from that, migrating to a newer database version usually doesn’t get easier the longer you wait. And if you do it every few years, you won’t get out of practice 
Yes, I don’t have a problem with needing to upgrade other things to use the latest version (unless they are core items that there may be other reasons not to update (eg other applications still using an earlier version php which is still supported).
I also don’t have a problem with an old version becoming “unsupported”. It’s when the old version starts nagging, or even insisting, that the user upgrades to a newer version that the problem arises. If I want to maintain my system using old software and old versions of its dependencies then that should be my decision and my problem.
PS I don’t think NC are doing this - or at least I hope not - its a general point.
I don’t know whether it’s an option for you, but I can state that we have several NC 22 instances running reliably with MySQL 5.7. Overall daily users ~200.
It seems that only some apps, e.g. the mail app, would anyway encounter problems because indices could be reattributed after an entry has been deleted. But we are not using the mail app and in other apps it has so far not occurred as a problem.