As per the docs in 24.2

"APEX_JWT APIs only support HS256 symmetric encryption algorithm for claim signatures. Asymmetric encryption algorithms such as RS256 are not supported"

But ultimately, APEX_JWT is an API layer built on top of the DBMS_CRYPTO package, which in 19c does not full public key support, and hence RS256 is not available.

DBMS_CRYPTO does get that support in 21 and 23ai, but 99% of customers are still on 19c.

Of course, there has been 28 RUs of 19c and hence DBMS_CRYPTO has continued to evolve in 19c as well, so hopefully it will come in due course to APEX.

Addenda: Ilmar Kern has a home grown one if you want

https://ilmarkerm.eu/blog/2023/10/validating-jwt-tokens-with-rs256-signatures-in-pl-sql/