Well, it all depends on individual preferences but for me, I don't see the need to remove the copyright notice because it's another way to give credit to the developers who created the software and that can even help others to discover the platform as well.
This is kind of an irrelevant point because if you were looking to start a forum, you could view the source of any forum and find out what software it's running. This goes for branding free as well.
Branding free, to my original assessment, only stops forum spam because spammers are running searches for forums and grabbing forum URLs... you can't search for HTML.
Why would users care what forum software is running for as long as they can communicate? Unless, the community grows and likes one forum over another, so they decide to migrate together.
It's more likely than not the intention to advertise the software to people that might want to start a forum (and get a ton of backlinks to support "forum software", etc. for search engine rankings). But, any "tech-savvy" (in which you should have the basic HTML skills to run a forum) user should be able to determine the forum software without a branding footer.
Without looking at the copyright header here, you can view the source:
HTML:
<!DOCTYPE html>
<html id="XF" lang="en-US" dir="LTR"
And should be able to tell off the bat what forum software it's running.
While developing ForumDirectory, I've only run into very few highly customized forums (less than 5) where I would be like, "This is running the link structure like vB, but it also has HTML elements that are seen in both Invision and XF... so what is it?

"
(Link structure is also a way to tell too... i.e., /1234-thread-name/, /123/thread-name, /thread-name.1234/...)
I have about 10 forums' code on every platform saved in order to compare 1 forum against to quickly determine software automatically (if it doesn't catch the immediate signals like
id="XF"
, etc.), from phpBB to myBB to Woltlab to XF1 and vB3, etc.