Is Stockfish 11 (level 20) beatable? I have heard many people saying, “Hah! I beat Stockfish,” and one saying, “I am the best chess player ever! I beat Stockfish.” So I wonder if it is possible, just to know whether I should try to beat it. I tried to play it once; I barely played 25 moves.
The answer is that they either played it on some very handicapped mode, they took back A LOT of moves, or they are probably lying. 

Stockfish 11 is currently rated about 3607, and that is in standard chess. Computer programs are typically even HARDER to beat for humans at even faster controls.

There is a reason elite humans have not played matches against computers for a long time: even the Carlsens of the world cannot beat them any more. According to the ELO winning expectancy calculator, there is currently a 745 point difference between Stockfish 11 and Carlsen, and per the chart, that gives the computer a 99.5439696% chance of winning.

Note: The ratings used above are from different rating pools, and thus, are not truly directly comparable, but used as a rough estimate of what would happen between the computer and Carlsen.