It looks like premium features are not foss, like SSO for example (but I might be wrong):

https://bitwarden.com/help/about-sso

> Login with SSO is available for all customers with an Enterprise organization

Only saw the reply late. All enterprise features seem fully open source. Eg SSO is in the source at https://github.com/bitwarden/server/tree/master/bitwarden_li...

Correction: that part of the source seems to be only 'source available' rather than FOSS.