ATT opt-in numbers are all over the map at this point: AppsFlyer was predicting at least 39% a few weeks ago, and I've heard a range of anecdotal reports anywhere from 1-30% over the last few days.
While Flurry's share of the app analytics market has definitely shrunk over the years (and it's arguable that some of Apple's language on the User Privacy and Data Use page is specifically designed to target Flurry's business model), their SDK is still widely implemented. That means their ATT opt-in numbers (currently sitting at 11% globally, and 4% in the US) reflect a better picture of the entire overall industry than reports from individual apps, but also aren't reflective of the opt-in potential from a truly optimized ATT implementation.
Keep in mind that iOS 14.5 adoption is still in single digits right now, likely indexed toward savvy early adopters who are probably predisposed against opting in.
Finally, this is a good place to point out that single-sided ATT opt-in rates mean little in isolation — the user must choose to opt in on both sides for anything useful to happen.