I have no way of knowing who has sexual health screening or not, but my suspicion is that it's very low on both sides.
For me it's incredibly important both for me and my unpaid life sexual partners, always gone regular as clockwork even though access is limited in my area.
Also had screening by post as part of some sex-worker research projects.
I also use the clinics for back up contraception, and other advice.
I suspect quite a few clients might (and I could be wrong) assume STI clearance by proxy, ie Ruth is ok so I must be ok.
Every so often someone will ask me either at enquiry point, or within a booking - 'Are you clean'. My answer is always I can never say for sure, testing is based on samples taken on a specific day, some of which will date back to previous few weeks due to incubation periods. All I do is use condoms for all intercourse with everyone, and have full including oral swab screening every time.
I'm also obtaining Gardasil 9 vaccine privately very soon, which will help reduce pick up and transmission if I ever bump into the cancer-causing varieties of HPV.
As above I'm not sure many people go to these lengths, although I hear many other sex workers can and do go more frequently due to easier access but they may also see more clients - advice is often based on activity.