Actually, there is an RCS API.
However, Google restricts its use to themselves and specific OEMS only. IIUC, that's currently only Samsung[0].
[0] https://www.xda-developers.com/google-messages-rcs-api-third...
IIUC, the RCS API is not part of AOSP[0], and as such, not part of LineageOS. As such, I believe the answer is "no."
Google has not provided any public API for RCS, and they control the ecosystem fully. If Google wanted other clients to use RCS as a platform, there would be public APIs that Signal, iMessage, Textra, Facebook Messenger and other SMS clients could integrate with.
All that being said...
I still trust Signal's Stories implementation over any other. While I believe they could have competed with SMS-capable apps like iMessage, Google Messages, and Samsung Messages, if pivoting into WhatsApp/Instagram/Snapchat/TikTok territory is what they'd rather do, then I believe they can execute it well.
edit: ah, they announced recently that they are removing SMS support in Android. The reasoning is solid IMO, I've accidentally sent insecure messages before.
For anything else, using SMS is like using Notepad to write books. Many better alternatives have come through in the decades since that tech was new.
It feels very weird to have both a very good end-to-end encryption (the Signal encrypted messages) and a very bad system (SMS) together in the same app. People should just move away from SMS, it's not like it's hard.
Did they not see any value in signal over SMS? Didn't you have any group chats?
Come to think of it, I'm having a very hard time coming up with any sensible use case for using Signal over text message. Presumably both sides still need Signal for the encryption to work, so what's the point? Might as well use the internet to send the message. The only scenario I can see it being useful is when you have GSM, but no internet connectivity, and that's rare these days.