Yes, some people might say that with modern Photoshop software, this can be removed, but I want to challenge them in doing that with documents that was written on and they will soon find out that it would not be that easy and it will also leave some new metadata for the forensic software to be extracted, if it was later used in some crime.
Also, when a hacker gets hold of say 100 000 KYC documents, they would rather use the "clean" documents, than having to go through the whole process of the "cleaning" of your documents with your own water mark.
If the site does not want to accept the "marked" documents, then you know they want to use it in a KYC scam. If they ask you why you "edited" the documents, you just say that you want to track where your documents was sourced from, if it was leaked. You can also keep a copy of those documents as proof that you used it at a specific site, so it will be easier for you to trace, where the leak originated from. (Photoshop will leave some traces of the editing that was done and you might pick up which document they used at the new site.)
Did you make that with big exchanges? Could you tell us which ones please? It would be very interesting for the community to know which exchanges are accepting that. TYVM