I'm trying to implement a solution for this situation:
There is a restaurant in which users can access RestAPI, using Google, Facebook, GitHub. There is even the possibility to login inside using the SPID authentication. In the first case I was considering Keycloak as a better solution for this case, but then i've read about Okta and its pros.
I cannot understand reading and searching on Google if SPID authentication is supported by OKTA. Looks like to be, because it should be using SAML, but I haven't found a clear solution.
I would advise going with okta or Auth0 due to their constant evolving product nature and the support they provide.
Regarding SPID authentication, I think you should be able to set up it up as Identity Provider inside Okta or Auth0 I will suggest getting a free developer tenant via https://developer.okta.com/signup/ or https://auth0.com/signup and trying to set up the the Identity Provider. Once you test and confirm, you can decide to proceed further.
Let me know if any further input is required.