As far as I'm aware, that feature was broken, and has to be repaired before we implement it back onto the site. In the meantime, you can easily search for players or servers on MCBans using the following links:
Thanks for taking the time to start this discussion, I appreciate player feedback and enjoy reading these types of threads! As for your idea, I don't see why we shouldn't have a search option to quickly search for players to view reputation, and other information about their bans, etc. I'm sure there was a valid reason for it's removal, but I'm not too sure.
Anyways, thats's my position on this idea; hope I've helped!
It was operational for a while however it was removed due to being broken (as Daph said). We're looking to bring this feature back in future however it's not a priority. Personally I found this feature useful as I'm incredibly lazy and I can't be bothered sometimes to type it into the URL bar.