Is this tire still usable?
 

...

From the looks of the wear on the other knobs and the almost worn away center knobs I'd say the tire sure doesn't owe you anything. No panic but it's time for new tires soon in any event. Make it a little sooner rather than later.

Rocks and sticks could cause a puncture easier at the torn knob site but I wouldn't call it a sure thing.

Many tire brands have problems with knobs tearing off. Kenda and Nokian are the two I see most often. Anyway, we have lots of riders who will basically continue to ride them until too many knobs rip off to maintain traction or the tire wears out, like yours is. Is the flat risk increased? Probably. Is it the end of the world? Nope.

it is okay if you for commuting, but not for MTB

MTB tires are cheap (at least some of them). I would be moving on myself.

Time to replace but also has time to find a deal on price.