Why would I ever need to support arbitrary Oauth2 providers? As long as they're following the standards, a generic Oauth2 client should suffice. If they're misaligned with the standards I wouldn't be using their service.
I can tell you never have, because there's no such thing as a generic OAuth2 implementation. They're all slightly different and incompatible. SAML is what you really want in terms of making things works more generically, but even that is generally a nightmare as you still need mappings.
I can tell you don't know about OpenID Connect. ;)