NC admin manual still lists Redis for transactional file locking. But Redis recently changed its licensing and (admitts) is no more open-source as per OSI definition, it will be excluded from many linux distros (i.e. Fedora is already disucissing this). Maybe it is time to look for some other solution.
Is there already some other working solution? I found KeyDB claiming to be “faster drop in alternative to Redis”, but I’m not sure if it can be used for TFL with NC…
As an end user, I wouldn’t do anything yet except wait and see how the various community forks turn out…
One of the most promising is probably https://github.com/placeholderkv/placeholderkv/ started by one of the Redis core developers.
Another fork, which has also changed the licence from BSD3 to LGPL-3.0, can be found here: https://redict.io/
The KeyDB fork, which has been around for a while and is still based on an older version of Redis, was initiated by Snapchat, and it looks to me like they developed and optimised it primarily to run their own services, so I have no idea how well it would work for transactional file locking in Nextcloud.
Also, since the project seems to primarily serve the needs of Snapchat, I have no idea how much they listen to the community and how eager they are to move to the latest OSS base of Redis, but there is some discussion going on about this on their GitHub: https://github.com/Snapchat/KeyDB/issues/798