An upgraded version of The Stanley Parable, fittingly named The Stanley Parable: Ultra Deluxe, promised players more choices (or lack thereof, depending on how you want to look at it) and new endings, to be released in 2019. When the end of 2019 rolled around, developers Crows Crows Crows and Galactic Cafe released the delay announcement to end all delay announcements, moving The Stanley Parable: Ultra Deluxe to "2020, to sometimes probably before summer, but it sort of depends on the specifics of how development goes".
It's now late August 2020, and while time is a funny thing that should not be trusted, that tentative release window seems to not have been met, which the dev teams acknowledged by taking to twitter and photoshopping, or probably just MS Paint-ing, some recent game delay announcements, such as Halo Infinite and Death Loop, to announce their own game delay. Since all of the recent game delays cite the same reasons, it's not unreasonable to assume that developing this game, too, has become difficult because pretty much everything else is.
The Stanley Parable: Ultra Deluxe Development Update pic.twitter.com/rdKzJA7yafAugust 18, 2020
The Stanley Parable will now release in 2021, probably. You can join the Crows Crows Crows Discord or sign up for the newsletter on their official website. If the newsletter is as good as the Crows Crows Crows Twitter account, it'll be worth it whether The Stanley Parable: Ultra Deluxe ever releases or not.