Thanks for the link. That's what I was missing.
On the subject of OIDC, there are several that could be flagged as supporting SSO with little or no extra effort:
Discourse: https://github.com/discourse/discourse-openid-connect
Mattermost: https://docs.mattermost.com/onboard/sso-openidconnect.html
Element: https://element.io/integrations/delegated-authentication