Toby Fox tweeted this out just now, along with some tweets as a sorta' development update:
UNDERTALE was released 4 years ago today... kinda feels like just yesterday though. I want to share some small things from DR but I want to do another small progress update first. The design of the actual game and the specifics of what happen are going really well. I have a lot of confidence in the game that is currently written in my notes. At least, I think people that enjoy the things that I make, will enjoy it. That's all I'm ever aiming for. There are other aspects that are facing some hurdles, but they'll definitely get overcome. Trust me when I say that, although I want to take my time making an excellent game, I also want to get the game into peoples' hands as quickly as possible. Seriously.
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UPDATE: