I think jackie_w has nailed it,
the class name and it's location and original purpose is stable, and it would only exist where a book has been sold in legacy mobi format, not in azw, and has then been converted.
so arguably it is a calibre conversion artifact and can be detected as such ?
i.e. I'd settle for removal only of matches for
<div class="mbppagebreak" id="calibre_pb_\d+"></div>
that's probably how I zap them in sigil: I had a quick look at my recent sigil find/replace but any like that have dropped off my recently used list