Sony had chosen not to renew its trademark for The Last Guardian in the US.
VG247 reports on the US office of patent and trademark website listing, which shows Sony has allowed the current trademark to expire.
What this means, if anything at all, is anyone's guess. Sony abandoned the previous trademark for the property to little consequence, and the company has reaffirmed as recent as this past December that the game is still in development.
Here's a capture of the listing: