Stefan Monnier writes:

In any case, the above is equivalent to just removing the `(and empty
block-comment-start/end)` because `comment-start/end` should never be
nil anyway, so it's probably worthwhile thinking about what we want to
do with `block-comment-start/end` here.
Maybe "nothing" is the right answer, of course.

block-comment-start/end have been unused for almost 20 years, comment-start/end had already replaced them ofc. So, I think block-comment variables should no longer be used to indent comments (as it seems that it was planned), instead use them for other purposes, give them a "new life".