The title here is burying the lede and should be updated to something like "Jellyfin founder, project leader and core contributer all leave the project".
It's not just Andrew [Rabert] leaving. This is an announcement by Joshua Boniface (the current project lead) and Anthony Lavado (a core contributor) announcing that they're leaving too.
Boniface's and Lavado's reasons are mostly akin to burnout, or the need for a family life, which I totally understand, especially as Boniface has been the face of the project for quite some time. I use Jellyfin daily, and I know exactly who Boniface is, I hadn't actually heard of Rabert, though I'm grateful for him founding the project. Boniface in particular has lead Jellyfin through huge levels of internal changes, bugfixes and improvements. It is throwing off the janky architecture of Emby, one part at a time.
Rabert's reasons are here: https://github.com/andrewrabert/jellium-desktop/issues/609
An obvious sore point is that Rabert has been AI-coding the Jellyfin desktop app, while the community (who bear the brunt of drive-by AI slop) have strongly opposed it. Even if he's a great coder and makes a wonderful desktop app with the aid of AI, that seems irreconcilable. Every wannabe contributor who shits up the project and wastes volunteers time with AI slop will say "you let that guy add his AI generated stuff, why not me?"
So he's leaving the Jellyfin project and he's going to rename his desktop app, to see if it stands on its own merits as a third-party Jellyfin client, rather than an official project. I wish the best of luck to him.
Even with that angle, it's true that the "Jellyfin community" (which includes all bug reporters, users on forums, etc.) can be very rude, directly to the contributors who may have inadvertantly added bugs. And Jellyfin is going through some growing pains, for example their 10.11.x series rewrote all the database handling to be a much better fit for the language (from raw SQL to .NET EF Core), which gives them a much more stable base to make changes upon, but it also had some performance regressions that rendered some third party Jellyfin apps unusable. (Those third parties' advice is not to upgrade). But at the same time, the project is getting there, supporting developers who are systematically fixing those regressions. The project has thousands of bug reports, but it also has thousands of fixes, and thousands of merged PRs. They are keeping up with the demand. I have every confidence Jellyfin 12.x will be great, and I'm looking forward to the new possibilities for plugins and enhancements it will open up.