@gotwf the database abstraction layer is fairly stable, we haven't had to make many changes to it at all for years. The Postgres implementation is fully supported 🙂
It would be nice to have a bit more documentation for the Postgres support. It's only mentioned here and here:config.json - NodeBB Documentation Fedora - NodeBB Documentation
If you follow the other OS install guides you already have installed MongoDB when you reach the config.json page.