One of the rules of Mario Maker is that in order to post a level, the creator has to clear it, to make sure that all posted levels are possible. But the creator of this level admitted recently that their clear of the level was tool-assisted. The team that was working on beating all the levels don't consider such "hacked" levels in scope.

Article is from a few days ago

So this map (Trimming the Herbs) wasn't the last to be cleared, and they have all been cleared now?

"Trimming the Herbs" was disqualified after it became clear that the original author of the level hadn't cleared it honestly (i.e. they abused glitches to upload the level without clearing it, or hacked the console to cheat through the level). Other levels had been disqualified for similar reasons; this one just took longer to confirm.