Part of the problem is deciding what the +1 should be attached to. Some people comment +1 just because they want to see a PR merged. Some +1s are intended to be associated with a specific comment.

But there's other systems for that; what you're describing are 'bumps' that have been in forums and such for forever now, and I'm sure that e.g. an upvote system (or sort by +1 reactions) would replace that system.

I think it could be done on a per repository basis, like, basic behaviour rules just like at forums and other such communities. It's then up to the maintainer / moderators / whatever to allow or disallow +1-style comments, and what the 'punishment' would be. They could then also tweak a setting that auto hides comments if they're shorter than X characters.

You can sort by reactions on the issues list, if that’s what you meant.