Ah, never mind.
Just tested a new Outline instance, with Google SSO.

Although, it works, it works only with Google Workspace accounts, and not with private emails.

So to not use Cloudron SSO you would have to reinstall the app and migrate the data manually, ensuring somehow the user mapping for outline is fine.

How to remove Cloudron SSO, from an existing Outline instance?

(I do already have another SSO configured, Google, in this case)

"A SSO login mechanism has to be configured to be able to login."

So while this is a somewhat strange behavior, this is what upstream has. So unless one wants other external auth providers, I guess Cloudron SSO is the preferred one.

I guess we can then remove the optionalSso flag and only have Cloudron OpenID integration.

I have just tried again and got this in the logs this time:
{"level":"warn","message":"Enforced https was disabled with FORCE_HTTPS env variable"}