I've removed the copyright footer on two XF licenses (and paid DBTech $300 or something for global on all their add ons for one forum).
There is a noticeable difference in spambot registrations with the copyright footer vs. forums without it. A forum with the copyright footer seems to be hit almost as soon as it's indexed in Google (as they can easily scrape new forums by searching that phrase, or for instance, "Community platform by XenForo"), while a forum without it can go months before seeing 1 automated spambot signup.
I've tested this across 4 different live forums and, since all use the same methods to prevent spam, I can only conclude it's the copyright footer that draws them in.
That said, if you launch with the copyright footer, it's probably not going to make a difference to remove it a year later because it'll already be added to spambot lists as a forum that can be spammed. But you will notice a difference when you launch one without the footer.
Prime example: FD was attacked almost instantly (copyright footer)... RS/ROFL hasn't been hit but by 1 or 2 automated bots (could be human) and 1 or 2 human spammers. FD was online for a much shorter period, but ROFL has been online since August 8, 2024 (nothing), and after the rebrand, and has yet to be attacked like FD.
Your forum (should) eventually become too big to ignore and get added to the list, making it moot. But, it's a lot less work in the beginning, where you could spend the time you would've been fighting spam to build more content.