To be honest, we're still finalizing our long-term pricing details. For what it's worth, "Sign in with Google" and "Sign in with Microsoft" are available to all users, and we'll prioritize additional canonical OIDC providers as demanded (let us know what you need!). 100% agree with the security benefits here. The SSO reference on our pricing page specifically applies to SAML-based IdPs, intended for customers who require further customization re: auth and provisioning strategy.
I hadn't the time to look around further, so I missed the Google and Microsoft SSO login options.
As long as those are/remain free, there is no SSO tax.
I will have another look when I have more time, thank you.