Apart from BOINC there are quite a number of forum (thread) do not support automatic subscription, indeed in my experience the norm is that one has to elect to be subscribed to a particular thread.

Only one of the 100 plus forum I use regularly has auto-thread subscription.
Yet again, if you think it's a good idea, the go over to Github and propose it.
Follow the link below, click on the "New Issue" button, and fill in the boxes as best you can. It is useful when making a feature request to say "Feature request" in the title. It's so long since I joined Github that I've forgotten if you have to subscribe/join to actually be able to post a request there, but that's not uncommon (and Github is not part of BOINC, but something much more widespread).

https://github.com/BOINC/boinc/issues

Strictly Github is not a forum, but is a "bug" and change tracker.
Don't expect chat in the same way as seen on a forum, you may get a question or two if people don't understand what you are asking for, and maybe one or two folks saying "great idea"/"stupid idea". Also it can take months to get the first response, as the developers can appear to be tardy particularly in the weeks immediately prior to a planned release.

I'm probably in about 20-30 forums, and there's isn't one that needs me to manually subscribe. The point is, why would you write something and not want to know there's a reply?

I am in one that automatically subscribes me to any thread I comment on and it is a pain in the proverbial! Any forum that has autosubscribe should have an option to turn it off so it doesn't have to be turned off for each thread individually!