Molly is a Signal client fork that uses UnifiedPush instead of Google’s push notification service.

Direct link to PDF of RCS Universal Profile specification

This PDF link can be found on a search engine. You can also fill out the form with fake information and a throwaway email to get the link, though you don’t have to.

The RCS Universal Profile is a publicly available standard, but there is no FOSS client stack that implements the standard. Google restricts RCS support on Android to the proprietary Google Messages instead of making it part of the Android Open Source Project. Apple Messages and iOS are also proprietary.

The EU should use the Digital Markets Act to force Google and Apple to allow competing RCS clients on Android and iOS.

I don’t think that’s the case. The v0.19.0 release notes say:

Instance Blocks for Users

Users can now block instances. Similar to community blocks, it means that any posts from communities which are hosted on that instance are hidden. However the block doesn’t affect users from the blocked instance, their posts and comments can still be seen normally in other communities.