The Editing team is working on mw:Talk pages project/Notifications. It is a way to "subscribe" to a single ==Section==. It's a bit like phab:T2738 but it uses Special:Notifications instead of Special:Watchlist. It will only work on non-Flow talk pages. Unless there are objections, I'm going to ask the team to offer this as a default-off, opt-in-only Beta Feature here. Once we have a better idea of whether it works reliably, it will (hopefully) get turned on by default for everyone, but that's probably months and months from now.
Topic on Project:Village Pump
I think it's a good idea. At least, if there are any issues it can be turned off, and will help identify possible issues early.
I have to note, however, that here on mediawiki.org almost all talk pages use Structured Discussions, so it may not have a lot of testing surface...
I don't see a problem with the proposal, but the above comment is also true.
Thank you all for your responses. It sounds like it will be ready in a few weeks. As you say, most pages are Flow boards now, but it might get some use on some old User_talk: pages.
We're on the schedule for the 18:00 UTC backport slot, on this coming Monday, 28 June. This will also be offered (default off) at six other wikis, including Meta-Wiki. See phab:T274281 for the rest of the list.