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.

Second this, I'd say started being common for Web apps in 2008 and "industry standard" by 2012 in the SF Bay Area.

I'm not sure it was as common as with Django, but it was a very common one, especially as Heroku only provided Postgres support.

~2011 I remember people religiously recommending postgres over mysql. ~2009 it seemed the go to db was still mysql.

Oracle bought Sun in 2010.

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.