I'm not 100% convinced we should revert our <hr> changes to that of v4. First, it'd create some breaking changes or deprecations for the $hr-opacity variable. Second, it'd be a breaking change to move from support for .bg-* utilities to .border-* utilities for customizing the element. Just feels like a messy option either way. The border solution does perhaps provide crisper rendering when zooming, but I'm not sure that's enough to make the change.
For now, I've documented something about the change in our migration guide since we missed an entry for that.
Thoughts @twbs/css-review?
/x-ref #34374 (closed) and #34292 (closed)