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.
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.