I am glad I still have it.
Previously if you had owned the P.T. demo file on your PlayStation 4, you could simply go to your Library and re-download it. But as confirmed below, the file is now completely gone - even from owners of its 'digital license'.
The normal series of events surrounding digital license holders on the PSN Store - for files that have since been removed - is that they disappear from the storefront, but owners could still find them in their download list for future use.
I believe this is the first case of a file completely being removed, regardless of ownership. The only way you can now play P.T. is if the file is already installed on your PlayStation 4 system.
Source: http://crashlanded.co.uk/konami-removes-pt.php