Maybe we just implemented the offline page because we started using the upgrade procedure »stop container, git pull, launcher rebuild« after being hit by things like [1,2,3] for a few times actually.
Maybe something changed on the robustness of killing PostgreSQL if it wouldn’t shutdown in time to run through the upgrade process smoothly.
Either way, the online upgrade (again) worked well for us when giving it another shot right now. So, nevermind and sorry for the noise.
[1] Discourse stuck on "Currently Upgrading" - #15 by amotl
[2] Upgrade failed due to unclean database shutdown
[3] https://meta.discourse.org/t/upgrade-failed-due-to-unclean-database-shutdown-ii/103268