In bs2 muted colour is .muted and in bs4 it is .text-muted. I think that would be easier to add .text-muted to theme/boostrapbase/less/moodle/bs4-compat.less rather than adding both .muted and .text-muted to all our templates.

If we don't want to do it for whatever reason we should still ensure that bs2 themes use .muted and bs4 themes use .text-muted

I've only done a quick check and:

git grep "class.*[\" ]muted" seems to be fine
git grep "class.*[\" ]text-muted" does not look fine as there are some boostrapbase appearances

The regexp should be improved to try to find more occurencies. Having a mix of bs2 and bs4 core templates does not help automating these checkings.