I think the "one sentence per newline" convention in modern markdown was actually made for reviewing markdown docs in github diff review, which defaults to softwrap.
I would love softwrap in the diff-reviewer since orgs, and I think "markdown editing in PR-approval diff-viewer is a major usecase for me. For example, at DIF we do SO MUCH markdown-in-git: all our collaboratively-written specs are basically just markdown files displayed in a custom static site generator, managed currently through github (which makes me really nervous as the rate and severity of GH outages keeps ticking up over time).
consider it done! we are presently reworking large parts of the appview, so this will be added to the new web client 😃