There are no foolproof methods for detecting leaked content on OnlyFans. There are no distinct codes or hidden watermarks, except for the ones added to the image by OnlyFans settings. Identifying the person responsible for the leak is challenging, unless they post something unique to themselves and not mass-distributed. Even OnlyFans themselves cannot take action against leaks, as stated in their Terms of Service. Their role is limited to assisting in the removal of leaked content. OnlyFans is not responsible for policing leaked content.
It's to provide the basis for when they file a DMCA request. The same thing as posting a No Trespassing sign. While it should be fairly obvious that you not wander onto someone else's property unless you have reason to be there, without that sign, you COULD argue that you weren't aware you were actually trespassing.
The DMCA request comes from Onlyfans terms and conditions which you're required to physically agree to, to be able to use the site. You cant add supplementary terms and conditions by simply posting them as a bio. So if the only grounds for removal was a few words in a bio, a user can simply say they havent read it therefor they havent agree'd to it and are not bound by them, so you cannot take the photos down.