Things are starting to require PHP 7.4, like the Maps extension. PHP 7.4 is available in Debian 11.
Should investigate if we can do in-place upgrade.
Docs: https://www.debian.org/releases/bullseye/amd64/release-notes/index.en.html
Pay special attention to: https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.en.html#idm1446 (exim)
Notes
Update went mostly fine. Small issue was caused by gem installation of librarian-puppet, that stopped working after the update. The gem version was removed in favor or Debian packaged version and the issue was resolved.
After the update there were issues with backups (fixed) and synclock (temporarily fixed, but yet to be fixed properly).
Outcome
Translatewiki.net now runs Debian 11. This gives us newer software versions with bug fixes and performance improvements. This including PHP 7.4 was a blocker for deployments.