One small but quite important question when looking at the operating system options at System requirements — Nextcloud latest Administration Manual latest documentation
Are Debian based operating systems (like the famous Raspberry Pi OS) supported too?
Not sure what the criteria for that list was/is. In my understanding Debian plus modifications for the hardware equals Raspberry Pi OS.
This page is about officially supported systems (I suppose that they are used by customers and that are regular tests). Ubuntu is Debian-based and supported as well. But probably most of these debian-like OS will work. Even other Linux distribution (archLinux) and non-linux like FreeBSD are known to work.
Only limit from NC 26+ is that you need a 64-bit OS!
Basically in regards to support, comes down to filesystem and if you can make PHP run with the needed/required libraries, aswell as a supported database engine and webserver. So Windows is out, but everything else remains. What is written in the docs, is which is known to be supported with standard libraries from OS distro repos and without need for much tweaks of the OS itself. And the listed supported OS is what you can expect to find official guides for. But it is not a list of “only works on these”.