I believe this might depend on Carrier if you are in US. I know someone who uses spectrum and is able to use RCS on his s10+. I have S21+ and use Lycamobile and have to use Google messenger to use RCS.
My carrier uses t mobile too and after they made us switch to new 5g Sim cards the settings in messages completely removed rcs chat options. It was working for me before I did the Sim switch. I'm so annoyed too! I want to use my theme I have on my phone and see when people are typing to me I don't think that's too much to ask!