Jack Tretton just disclosed this during his interview on Gametrailors with Geoff Keighley.

At this point, I am not sure if we will ever see this game come out.

That

is a tremendous bummer. :(

http://kotaku.com/sony-corrects-themselves-says-the-last-guardian-is-not-512720979

Yoshida says it's not. If it's not at TGS, that's a different story. I don't think you should be too worried for the time being.

This is the quote, if people don't want to spend time on Kotaku:

"That hiatus term—I’m not a native English-speaking person—but that’s misleading," Yoshida told me a few minutes ago during an interview here at E3. He'd watched Tretton's comments on Spike.com while eating lunch. "The game is in active development. [Lead designer Fumito] Ueda-san shared updates a few months ago. Nothing has changed. My answer is, 'Yes, the game is in active development.' It’s not hiatus at all. But we are not ready to reintroduce it. When we are ready, we will do that. Please wait."

I wonder how well it'll need to sell to cover the cost of so many years in development.