Many places also (used to) have sign saying 一見さんおことわり which roughly translate to "first time visitor not allowed". A little bit up to interpretation but usually in the scope of:

* store only for patrons, but welcome if you come with patron.

* anyone welcome but if you're only coming once (tourist etc), please don't (destroy the vibe).

For obvious reasons foreign tourist couldn't get this so many places just put up a "no foreigner" sign. You'll still see local foreigner sometimes hang around those places though

This is false, IME. If you spent enough time to learn how to navigate it, you could get in to most of them. I'm not even fluent and it really wasn't a big deal.

I did this living there from ~2009 - ~2016. Wasn't an issue in my visits afterwards either, at least up until the COVID years.

I will say that when I go back each year (1x/2x per) post COVID, I've seen more of them trying to be firm on it though - presumably due to the tourism influx.

I have been to many snack bars. They're everywhere, they're not "seedy", and nearly all of them are open to anyone.

That may be true, but I don't think that's what they were talking about. They were talking about fairly normal izakayas

I've been to many snack bars, and once in my almost ten years in Japan, I found one that didn't welcome foreigners.