When I started using Django 10 years ago during the Django/Rails era, Postgres was by far the most recommended DB to use. Not sure if that was also true in Rails community.
I definitely recall being on the talk committee one year for DjangoCon, and there was some rough discussion. (Context: The conference was generally a two track conference but keynotes and a few other sessions were single track). One of the single track talks was about Postgres. The discussion was roughly "If we have a Postgres talk we should have another talk like Mongo or MySQL" and the response was roughly "Everyone in Django is using Postgres and if you're not you should be at the talk to learn why you should".
Way more Rails apps used MySQL or other databases, it was largely Heroku winning Rails that led to the strong adoption amongst that community.
Started using rails 10 years ago, myself. Postgres was by far the most common recommendation. In fact I learned a rule of thumb, early on, that I abide by still today: Always start with postgres and only migrate out once you understand your data well enough to consider the migration a clear win.
