We currently use SOAP for authentication (a custom PAS addon) and that works well enough. We don't have control over the authentication source so switching from SOAP is not an option.
We want a solution that will allow our users to sign into Discourse and authenticate against Plone (or maybe the same SOAP).
In SSO you almost always have concepts of identity providers (idp) and service providers (sp)
If discourse is your idp, which standards does discord support in the role of an idp? Plone supports most standard in some form. So I would further researcg what Discord has to offer when it has the “leading” user/group database.