[Update: So, as noted in the comments below and now by developer Toby Fox, Using the Deltarune uninstaller could possibly wipe files from whatever folder the Deltaruneuninstaller is located. Do not use the uninstaller, delete the Deltarune folder itself. An unfortunate epilogue to what is otherwise a clever bit of marketing.]
The official Undertale Twitter, which has been in blackout recently, has just updated with a repetitive link, which leads to a website and a downloadable .exe file known simply as SURVEY_PROGRAM.exe.
The download page merely offers a few simple warnings, written in stark capital letters and in keeping with the old-school text font of the popular and dramatic RPG. "There may be moving or flashing imagery... You may have to override security protocols to use the program... For public safety, you are advised to refrain from discussion of the program for 24 hours."
This ends with the ominous message: "You will accept everything that will happen from now on". The mysterious program can be downloaded now from Deltarune.com. Buyer beware, guys 'n' gals, we take no responsibility for what may happen... The perfect day for it though... right?