Subtitle ASS don't works

[Efficient_Good_5784]

You don't have to burn in the subs to all of your videos. That's what transcoding is for. The subs should get burned into the video if they're not supported on the ATV app by default (unless you disabled transcoding permissions which seems like you did).

I hope you have a server-side solution that is not to re-encode the subtitles of thousands of Movies.

Jellyfin does. Transcoding happens whenever a stream is unsupported by the client.

My Jellyfin server is based on the latest version 10.9.7 that works on Raspberry 4.

We recommend to not use any Raspberry Pi to host Jellyfin due to it being weak for transcoding purposes. You could use them as long the media direct plays, but we're talking about ASS subs here (they don't have major support in a lot of video players).

Given that I have already read several posts on the subject and it is clear that you have removed support for ASS subtitles and you have also removed the ability to change the Player in the APP.

ASS sub support was not removed intentionally, the experimental video player that could direct play it was.
Check these out for the reason LibVLC was removed:

https://github.com/jellyfin/jellyfin-and.../pull/2534
https://github.com/jellyfin/jellyfin-and.../pull/3639

Support for ASS subtitles will come in the future (no estimated timeframe).
There will be work done to have the current video player (ExoPlayer) in the ATV app to support ASS subs.

Forgive the outburst, it is not to criticize your exceptional work, but after the latest update of the Android TV app I find the Jellyfin Server practically unusable for more than 50% of the contents! ... how can I solve this problem? ...

You can always hook up a PC/laptop to your TV with an HDMI cable. Then use either Jellyfin Media Player or MPV Shim to direct play ASS subs (they both use MPV which supports ASS subs).