Patroni does have synchronous_mode_strict setting, which may be what you're looking for:

This parameter prevents Patroni from switching off the synchronous replication on the primary when no synchronous standby candidates are available. As a downside, the primary is not be available for writes (unless the Postgres transaction explicitly turns of synchronous_mode), blocking all client write requests until at least one synchronous replica comes up.

https://patroni.readthedocs.io/en/latest/replication_modes.h...

edit: seems I missed this discussion on twitter: https://twitter.com/jepsen_io/status/1265626035380346881