I participate in a subreddit focused on one particular modded mincraft launcher and set of modpacks (r/ftb, for the Feed The Beast launcher). Posts about modded minecraft in general are welcomed; /r/ftb is the largest modded minecraft subreddit, and there's a lot of overlap in terms of contents, users, mod authors, etc.

Similarly, I've never seen anyone upset about people discussing non-Javascript languages in /r/javascript; there's specific subreddits for some popular variants and libraries (like /r/coffeescript and /r/react), but nobody cares. The rules do say posts should be at least indirectly related to javascript, but a post talking about a better language would be seen as easily related enough.

/r/bitcoins rules do not follow the norms for Reddit communities, as far as I've seen.

As a counter-example, there are lots of tightly-modded subs that I follow that benefit from strong moderation of off-topic posts. Such subs include /r/personalfinance, and /r/askhistorians.

> I am sure promoting dog ownership over cats is frowned upon by /r/cats readers, too

You are wrong, nobody on /r/cats have gotten their comment removed or been banned for talking about dogs or any other pet on /r/cats.