There have been dozens, called anything from Message+ to Joyn. Most seem to be shutting down, though, because nobody used RCS since its inception from somewhere around 2009 and renewed interest in launching RCS services around 2012.

There are a few implementations on Github. https://github.com/android-rcs/rcsjta seems to be the most complete but uses an older version of the spec. https://github.com/Hirohumi/rust-rcs-client is a modern implementation.

The spec is here: https://www.gsma.com/solutions-and-impact/technologies/netwo... if you're looking to implement it yourself.