The WordPress.org support forums now have subscribe by email, to tags.
To use, login, then view any single tag page. At the bottom of the page, sorta hidden on the left side, you’ll see a link called “Subscribe to Tag via Email”. It’s a toggle.
Note: If you subscribe to heavily trafficked tags, you’re likely to get a LOT of emails. I suggest you use this sparingly, or get an email system that can cope.
Note the second: I fully expect there to be bugs. This is new. Expect fixes when I find them.