The change in moving the item registries to its own class breaks backwards compatibility with any mods that access static fields previously found in Embers' RegistryManager class.

It may be a good idea to re-implement this class with fields linking to their respective registries.

We are aware of that. That's why we keep a list of all the mods we fixed compatibility issues with.
If you don't want to do it yourself I can publish a fix for it tomorrow. Nonetheless, thanks for the heads up.

It's for a new mod of mine Survival Tools. I can add the compat myself, but just wanted to mention the issue as the change in your fork breaks pretty much every mod accessing that class.

Please report any broken compatibility you find. We already have an extensive list of mods were we re-established compatibility.
Moving the registration of blocks and items into their own classes was a choice made to declutter the main register class and we won't add it back. We much rather implement specific compat on our end if a mod breaks with our fork.

I'll handle the fix myself since my mod is still in development and the Mixin fix can easily break if I have to change the handler for some reason.