This is an interesting one because I wasn't sure if YouTube actually collects any data from if a watermark is actually clicked or not
But if you look at subscription source, there's a section for interactive features (including end screen AND branding)
YouTube does report these changes on a daily basis as an option, and theoretically if the ONLY interactive feature you change on a daily basis was branding watermarks (and that's how thumbnail A/B testing works) that would indicate on some level if that would make any différance.
The only issue as to whether it's worth worrying about, is if there would actually be enough data being able to be gathered to really determine if there was a change in either situation. And there's still the reporting of end screen subscription buttons that would interfere with that, so I'm not sure how accurate it could be.
BUT if it's possible, I'd indeed love to have it from a detailed analytical standpoint. Not sure if enough people would use it to warrant it, but it would be great to bring more focus to largely ignored subscription source.
Personally I love questions like this that get really deep into the details that others don't generally think of.