You have to be careful about context. "one left" can mean "one is left" or "there is one left" but in this case "left" is an adjective and this is not a complete sentence It's possible that "left" could be a verb in which case the meaning is different. For example: Nine people were in the room. One left. There are eight people in the room now.

"left" here is not a verb. . How many sweets do you have left? I have one left. How many red ones. I have one red one.

"Left" can mean "remain", "remains", or "remaining" in this situation. You never change the ending for this definition. "One left" means "One remains". "Two left" means "Two remain". The two examples above are not complete sentences. "One is left" means "One is remaining". "There is one left" means "There is one remaining". "One was left" means "One was remaining". "There was one left" means " there was one remaining". "One will be left" means "One will remain" or "One will be remaining".

Be careful not to confuse the two meanings of left: departed and remaining. One ticket left = one ticket remaining. I left the room = I departed. Those are two completely different words.

You are correct. It is just a shortened version. Technically incorrect, but often used.