My first thought as well, but my second thought was why wouldn't they own up to it? Surely they know that owning up to something like this earns them much more respect and positivity from the community than "taking it down" for no good reason, or worse trying to cover it up.

(a) that's a lot easier to say than do, fear is powerful and (b) no, they wouldn't get any respect for Not. Having. Backups.

No, my guess is it was hacked so they shut it down. That is probably why they say "security risk"

I doubt this.

"After over a year in maintenance mode, the official Unreal Engine Wiki is now permanently offline."

It seems to have been a problem for some time.