According to Archive.org, the difficulty meter was still live on their site as of November 23rd, 2017 and gone on December 5th, 2017 (and thus it was removed sometime between those dates). As you can see from those two links, they also used to have something telling you the word's score in Scrabble and Words with Friends, which was also removed (though at an even later date). If you still want the information, you can check Archive.org.
As of 2022 (maybe earlier), it looks like they've added back this information. Some entries "show grade level based on the word's complexity". For example, premillennialize is "post-college level".
I remember looking for as much information as I could on this pre-2018 and finding no information on how they actually came up with the difficulty whatsoever (thus preventing me from trusting it). I also don't think the difficulty information was available via any API (which would have been nice news for programmers).