Jellyfin brings and uses their own ffmpeg-version which is not impacted by Synology removing hardware acceleration support, at least according to the following page.
Community Workarounds and Tips
- Use Third-Party Media Servers: With the removal of Video Station, some users are turning to alternatives like Plex, Jellyfin, or Emby. These third-party apps offer more robust media management features and support a wider range of codecs.
https://nascompares.com/news/how-to-restore-video-station-and-hevc-support-on-dsm-7-2-2/