I'm not sure how statistics track it, but when you're moderator on a subreddit you can see how many visitors use old reddit.

Last month for mine it was 1%. Now. This could be more due to their tracking not working for most old reddit users (I assume most have adblock), but it's unlikely to be a lot more.

The large majority of Reddit users are on the app. I'm honestly surprised they haven't killed old reddit off yet, and yes, I've used it myself for the longest time until I started missing some features.