I'd expect it's a large number, because I'd expect that most of the people who like the old UI also turn off telemetry.

When Ars Technica did a redesign a couple of years ago, they didn't launch with a dark mode because their telemetry didn't indicate that many people used it. There were a bunch of comments from users asking for dark mode, so they polled subscribers and the numbers who used dark mode were massively higher than the telemetry indicated. Turns out that the people who used dark mode were also the kinds of people who used strict ad blocker profiles which blocked the telemetry

It seems more and more like telemetry is mostly good for seeing only what your most clueless users are doing. It's a problem when your userbase is largely technical and has no problem finding the "don't spy on me" checkbox.

Projects with highly technical users would be better off listening to forums. It doesn't seem like the folks in /r/Thunderbird are overly concerned with the UI

Forums have their own problems. Most turn into echo chambers where one particular opinion is dominant pretty quickly. There's plenty of topics where one subreddit will be broadly positive about a subject, and another subreddit will be broadly negative about the same subject.

I'm not sure there's currently _any_ good way to get unbiased feedback from users. Ideally you should probably cross check a few different methods, but that's hard