Quite a few things implement discovery. Most IDPs advertise metadata, and many RP libraries consume it by default. IDPs even motivate them to consume it by rota

Quite a few things implement discovery. Most IDPs advertise metadata, and many RP libraries consume it by default. IDPs even motivate them to consume it by rotating signing keys pretty often. Not many things support dynamic client registration, which means the list of IDPs a site can use are pretty static.