Pkce is trivially easy to announce support for, you put it in the issuer metadata.
code_challenge_methods_supported
code_challenge_methods_supported
Though how would you implement it? Like, user comes to your website and wants to sign in with some foo.bar provider, do you force the user to paste in the domain where you go look for the metadata? What about facebook or google, do you give them special treatment with prepared buttons or do you force user to still put in their domains? What about people using your flow to "ddos" some random domain...?