GitHub does indeed have this this feature, and I’ve found it extremely useful over the years when making, reviewing and suggesting changes on documentation, website and other prose-related content. Unfortunately, as you might expect, it doesn’t go so far as to parse and dynamically reflow reST content in order, so the reflows resulting from arbitrarily hard-wrapping text makes it almost useless for such documents, whereas it is far more useful with content with OSPL and moderately useful with SemBr (though it essentially equalizes what would otherwise be SemBr’s nominal advantage over OSPL in terms of diff granularity).
I usually try to do this, but unfortunately it requires digging into the commit history when reviewing, which also means that GitHub’s suggestion feature, which I make extensive use of in PEP, docs, etc. PRs, doesn’t work at all. It also doesn’t solve the increased likelihood of merge conflicts, the final squashed commit diff is still equally noisy, and it still means one needs to do the work of reflowing at some point.