Many headless-browser stealth techniques involve rotating between the signatures and reflected metrics of real — but niche and/or ancient — User-Agents. (For some reason, the developers of these stealth systems think that variety beats commonality. Maybe it makes sense if they're specifically trying to overcome Apache mod_security's signature-based UA blocking or something.)
It turns out that when you actually see one of these UAs in your server logs, it's far more (99.99%) likely to be a stealthed bot that picked that UA out of a bag, than it is to be an actual niche/ancient UA.
In the case of the niche UAs, this is a tragedy of the commons.
In the case of the ancient UAs, though, there's no downside to blocking them entirely — because if the traffic is going through Cloudflare at all, then you're already requiring of the client a minimum version of TLS that the real old UAs can't even speak. So the only things actually saying they're that old device — but managing to get through an HTTP request at all — are stealthed bots.
