Thanks for bringing up the topic.
As far as I know, OIDC/SAML is the only way to identify users.
Inclusion in OIDC/SAML on paid subscriptions is a common occurrence, for example:
But, all these services have an alternative, access to the table of users and roles and a login form.
If you went in this direction, but did not provide an alternative, this, in my opinion, is not normal. Moreover, there are requests for this feature here.