I operate a service that measures this (see another comment on this discussion), and all I'll say is you'll be very surprised how many bots actually execute JS, especially stealth bots. You have to be careful either way.
I don't have access to the raw log files from the customers, so can't give you a percentage. All I'll say confidently is that my service processes a lot of bot traffic that needs to be filtered out before reporting.
BTW, are you the same Peter Hartree on this Segment thread? https://community.segment.com/t/1889n1/how-common-is-client-... It would appear we've crossed paths before on this topic. Please do email me if you want to talk properly. That Segment thread has my email.
I operate a service that measures this (see another comment on this discussion), and all I'll say is you'll be very surprised how many bots actually execute JS, especially stealth bots. You have to be careful either way.