Only Messages was allowed to support it. At the time it was written, for example, it was years after the Signal developers had asked for basic RCS support (2015) and given up on Google relenting and allowing it in 2018.
Signal has been rather dismissive of RCS in general. Of course RCS was as insecure and unencrypted as regular SMS/MMS until now, but I haven't found a sign that Signal ever even looked into supporting RCS at all.
At this point I think they just wanted to get rid of their SMS backup feature and used RCS as an excuse.
Signal was dismissive of unencrypted messaging. Google used their protocol for their proprietary E2EE system so it’s unlikely that this objection would apply if that system was open.
Put another way, if this was a Signal thing what other Android app is doing this?
None, but that issue has no mention of what you said in the other message
("Only Messages was allowed to support it. At the time it was written, for example, it was years after the Signal developers had asked for basic RCS support (2015) and given up on Google relenting and allowing it in 2018")
If they seriously wanted random third parties to implement it, anyhow, they'd have published a specification, not an "overview"