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.
/r/bitcoin has never been a venue for discussing alternative cryptocurrencies. Due to reddit's lack of a concept of sub-subreddits or the ability to have a subreddit's front page filter out a certain tag, the only way to remove floods of people discussing off-topic subjects is to say "sorry, please discuss that in another subreddit".
If you want to promote Ethereum, you can do so on /r/ethereum/.
I am sure promoting dog ownership over cats is frowned upon by /r/cats readers, too.