eldersc0de the "copyright" itself is shipped with the code as part of the
LICENSE file. MIT license doesn't require displaying any license for using the software, only for distributing.
As for a "powered by Flarum" text, I'm not sure whether there is any historical discussion about that. I'd guess there's just no proper place to put it. Due to the infinite scrolling nature of Flarum, footers are the exception rather than the norm.
I think Discourse also doesn't ship with any automatic credit by default? At least I can't see any on their community/test websites.