Nephalem Rifts give an extra legendary drop bonus as said on http://www.diablowiki.net/Nephalem_Rifts . I'm wondering after Rift Guardian is killed and progress bar disappears, if we continue to loot in the game, is the bonus still available?
Yep. The bonus is for the Rift. Not for the Rift until the boss is dead.
That said, Greater Rifts are going to have all their loot backloaded into the guardian themselves, as they will be timed. No loot will drop from anything at all except the guardian. In that case, the bonus will not apply, as only the guardian will drop loot.