A few more details are in the support article linked at the bottom of the post: <a href="https://support.google.com/feedburner/answer/10483501" rel="nofollow">https://support.google.com/feedburner/answer/10483501</a>. In addition to email subscriptions, they also mention that they are shutting down browser-friendly subscription pages and password-protection.
Annoyingly I still have one old feed that has a few hundred subscribers through it on FeedBurner, from way back before I decided I would always set up a custom domain so I could move off services like FeedBurner after they'd been Google'd.<p>Not sure how many of them subscribe via email, but I know that was one of the main reasons why some people choose to run their feeds through FeedBurner back when it was actually a thing.