Most sites have a preset for the major identity providers, Okta, Microsoft, OneLogin, etc. The ones who want to be nice will also have a generic "SAML" option allowing connections to just about anything - but that's when the home grown solution pain shows up.

But you can support just 5ish SAML IDP's and get a large majority of the market and have effectively no ongoing support issues because its really well known (and well documented) how to set them up.