We don’t have any clients that run Open edX on Postgres, but… it would be nice to leave that door open.
I dream of a world where we can run the stack with only a single relational database service (and without mongo… unrelated thread, but here’s links in case people want them, see Store modulestore’s course indexes in Django/MySQL and Replace cs_comments_service with pluggable alternatives). One of the working discussion plugins is Discourse, which requires Postgres, and it would be sp fantastic if one day we could run both Open edX and Discourse on the same sql db service.