Ok, git.platypush.tech has successfully been migrated from Gitea to Forgejo.
It didn't take long - it is indeed a drop-in replacement, but the systemd configuration packaged with Arch required a bit of tweaking to point to the previous Gitea paths and some permissions changes, *only then* it's an actual drop-in replacement.
Luckily I've always used `git` as a username rather than `gitea`, so that part, after a few changes in the configuration, won't require any more changes.
Tip: never, ever use the default username provided by your code forge (gitlab, gitea, forgejo...). Code forges come and go, and one after the other they are doomed to enshittification. Code repositories, configurations, scripts and documentation are there to stay instead. So never use a username that is tied to your code forge, or you'll have a hard time changing it later.