It looks like every time I go to check their stuff there's less of it, and they themselves seem to have disappeared from the face of the internet. what's up with that?

I really don't know what Hypnofood's deal is, but in my experience this seems to be pretty common among nsfw artists at least in this community, they'll post stuff regularly for a long time, then suddenly purge everything, then if they come back it's almost guaranteed they'll purge their stuff again someday, so save save save is all I can advise. It's probably dependent on the situation, but some things to take into consideration could be, family/friends finding out they draw stuff like this and not approving (this fetish is pretty niche and considered weird/creepy by most I'd imagine), mental health issues, depression, low self esteem, mental breakdowns, etc. can all cause an artist to get rid of everything, either cause they don't think it's good, or they don't want an online presence anymore, and finally, wanting to "go legit", if you want a legit job as an artist for a not nsfw piece of media or at a company, you probably aren't gonna want your employer finding out you draw weird porn art of dubious consent at best, so some artists purge their stuff in an effort to put their horny art behind them.

In Hypnofood's case? I honestly think they have self esteem issues, even though a lot of people like their art, they've changed their style drastically, and if I remember correctly, feeling their art wasn't good was what caused them to do their first few purges on deviantart and stuff. Now though? I almost think it's become a routine for them, come back online, post a few pieces, commission a few more, then delete it all. If you aren't saving the pics you like from them at this point I don't know what to tell you, because there is an almost 100% guarantee they WILL delete it eventually.