It is with great excitement that I can announce "subscribe" comments on Drupal.org issues are now dead! Long live issue following! Issue e-mail notifications are also improved.
Almost exactly 6 years after #34496: [meta] Add Flag module to allow users to subscribe/unsubscribe without posting a comment was originally posted, this feature is finally done and deployed on Drupal.org. If you notice any problems, please go to #1306554: QA for issue following on Drupal.org and comment there.
In the past, you had to comment on an issue in order to keep track of it, commonly done by posting "subscribe" (or variations thereof). Popular issues gained plenty of such comments, making it hard for contributors to distill the important and useful information in an issue.
You are now able to "follow" issues by clicking a button, without commenting on them. You can also "unfollow" issues, even ones you had to comment on but you are not really interested in. Lastly, you can now configure for which projects and issues you want to get e-mail notifications.
First, I'd like to thank the sponsors that made this possible:
This sponsorship allowed 3281d Consulting to focus on this project and get it deployed ASAP.
There are two big inter-related changes as part of this effort: how we interact with issues on Drupal.org, and changes to the issue e-mail notification functionality.
The most obvious change is that when you're viewing an issue on Drupal.org, you will now see a large green "Follow" button in the upper right corner:

Clicking the "Follow" button will use AJAX to flag the issue as one you are following. This will trigger two things: 1) the issue appears in the "Your Posts" and "Your Issues" lists (both on your dashboard and the separate tabs on your profile), and 2), the "Follow" button will be replaced with "Following", to indicate you're now following the issue.

If you hover over or click on this "Following" link, it will turn into an "Unfollow" button:

You can even unfollow issues if you wrote them or commented on them. Whenever you click the "Unfollow" button the issue will disappear from your tracker and you will stop getting e-mail notifications about it (if you get e-mail notifications at all).
You can optionally configure Drupal.org to send you an e-mail notification about updates to issues you care about. In the past, this functionality has been somewhat hidden, so a lot of users do not make use of this feature. Now, there is a centralized page to manage all of your issue e-mail notification settings, on the new "Notifications" tab on your account profile page:

This page lets you opt-in to e-mail notifications for issues on Drupal.org. By default, you get no e-mail at all. You can define both a site-wide default and per-project overrides. A very common configuration would be to just set the Default notification to "Issues you follow":

If you maintain some projects on Drupal.org or are otherwise particularly interested in their issue queues, you can also specify per-project overrides of the site-wide default. So for example, another common configuration might be to get notifications for all issues you follow, but to also get notifications about "All issues" in a few specific projects you most care about:

In the past, there was no way to define a site-wide setting, so users often had to configure e-mail notifications across a large number of projects. If you used to be subscribed to 50 or more projects with the "Own issues" setting, that was converted as part of the deployment into a single site-wide default.
The final change to the e-mail notification functionality is that you can now customize the contents of the e-mails themselves. Previously, you always got the entire issue history included in each notification. If you expand the "Configure e-mail contents" fieldset, you'll see the ability to only get the new content in each notification and some checkboxes to control what appears in the subject line of the messages:

There are more ideas to further improve the issue e-mail notification experience, so be sure to read the "Future work" section below.
While the financial sponsorship was critical to allowing me to spend the time I needed to drive this home, tons of other people contributed to help make this a reality.
As exciting as all of this is, of course there's always more work to do. Generally, we've been using the flag integration issue tag to keep track of things related to this effort. See also the drupal.org notifications tag. You can also check out the Expand "follow" functionality on Drupal.org community initiative page.
Unfortunately, I'm going to have to return to other commitments, so I can't just continue to pour unlimited time into getting all of these issues done, UX reviews, code reviews, etc. So, if anyone wants to step up and claim any of these issues to drive them to completion, that would be greatly appreciated.
Thanks!
-Derek Wright (dww)