[jraph]

But please don't make me create a Google, Facebook or Microsoft that all come with arbitrary ToS, auth schemes, tracking, blocking that would cause me to lose access to the forum and absence of support. Please accept custom OpenID providers.

[paulmd]

If spammers can just create their own provider, that would kind of remove the point of having IDPs act as a spam gate.

That's not a firm "no" but like, you are transferring the user-reputation problem into an IDP-reputation one, so allowing arbitrary IDPs is not going to work. If you allow federated ones, you need some whitelist or reputation network of which federated IDPs are the good ones.