Implementing some interfaces for compatibility is now impossible for soft-dependencies because it is no longer possible (to my knowledge) to specify that an interface should be implemented conditionally.

What I'd like to suggest is to move interface APIs to Registry-like types.
It may also be possible to make some APIs into capabilities, but such a thing is only viable on tile entities as there is to my knowledge no way to attach a capability to a block.

Advisory on this problem is very welcome.

Closing as this is mostly mitigated now with usage of forge caps/fabric api lookup. There's probably a long tail of stuff to be migrated but feel free to submit PR's for them and we can get them landed during a major version boundary

You could mixin your own classes, if Botania is loaded, now that Forge ships Mixin by default.