For those who missed out on the P.T. demo PuniTy is as close as it gets
A recreation of the P.T. demo developed in Unity has been created as a showcase piece for a development workshop, and you can download it if you like on PC.
Called PuniTy (thanks, Destructoid), Alberta-based indie developer Farhan Qureshi partially re-created the demo over the course of 104 hours.
Know going into it that Qureshi's demo is not a full remake and some of the puzzles are absent along with the narrative. There are also changes to the demo such as players starting out with the flashlight.
Still, for those who never tried P.T. on PS4 before it was pulled by Konami - or those who deleted it- it's as close to the original demo as they are going to get.
You can read up on Qureshi's development process through his website which also has a link for the download.