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.

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.