As a German I was really surprised that this isn’t something that’s done everywhere. Personally I’ve been running a Stammtisch for Japanese for 15 years now, and taken part in 2 others for over 10.
It’s really not complicated. You ask the bar owner whether you can have a table at the bar every month, and how many people will come, that’s it. If you do it Monday-Thursday it’s better for them, because the bar will be emptier and they will know how many waiters to call in. They might even make drinks cheaper for you.
There are different meanings to Stammtisch. What you describe is more like a regular meetup around a special topic. Those are not marked as Stammtisch in the pub, altough attendees call it like that.
The other meaning is high alcohol consumption and extreme populist opinions. That's the Stammtisch of the pub.
I don’t think that was ever true. A Stammtisch is not primarily about drinking, with something more refined on the side as an excuse. But I can imagine raging alcoholics like to believe that.
It’s really not complicated. You ask the bar owner whether you can have a table at the bar every month, and how many people will come, that’s it. If you do it Monday-Thursday it’s better for them, because the bar will be emptier and they will know how many waiters to call in. They might even make drinks cheaper for you.