PR isn't a place for discussion about what or how to implement change in the first place, that should be forum/mailing list/issues

and there is open issue for that discussion https://codeberg.org/forgejo/forgejo/issues/8634

#8634 is specifically about a breaking change that occurred in v12. It's literally the first line of what you linked:

> In the v12 release of Forgejo (fixed in v12.0.1) there have been breaking changes that impact third-party authentication sources that use Forgejo as a provider. If you have been affected, please help us assessing the impact ...