From my own history of proposals (as freelancer/agency) it seems to be about 20% which sounds reasonable. As a client on the other hand, have myself posted 49 projects and hired only on 17 - for the rest there wasn't a single capable applicant on only super-overpriced ones. So 35% and i always posted with definite intention to hire, and only on Upwork (out of freelancer sites that is, ofc i also used my own network). If someone has profiles on several freelancer sites to source people, 20% hire rate sounds like reasonable and not something Upwork needs to hide.
I was curious, and so I checked my own numbers as a client...
It says I have 100% hiring rate, but if I remember correctly, there were a couple of writing jobs that I posted and never had a workable applicant for. I'm not sure if this number is completely accurate. But I have hired freelancers for MOST jobs that I posted.
Of course I don't think that this reflects the overall Upwork hiring rate.
I think the question is - in one sense - not relevant - because every freelancer works in specific job niches. So the important question is what is the hiring rate within a particular job skill niche and pay level.
I didn't check 'hiring rate', instead actually counted my job posts and number of applicants. Yes hiring rate is wrongly calculated, i already reported this issue to Upwork support today.