Although not covered in the USATT rules specifically, foot stomping is currently allowed. There is even a question about it on your Umpire Written Examination.
In an older time, the foot stomp was illegal. Currently, it is allowed to stomp on the serve, even if it is much more distracting loud and shaking the venue floor than a loud grunt on serve.
In a sanctioned match, during the OPEN final, I made a shot that hit the net and dribbled out, I raised hand and said sorry, but opponent barely got to the ball and almost saved it. Opponent wasn't happy I said sorry, but the referee himself was umpiring the match and allowed the point to stand. I see where my opponent was coming from, but my reaction came from concern of good sportsmanship, so I think that aspect was under consideration in the umpire's thinking.
On a different note, in the same Open Finals match on game two, captured by someone on MyTT, my opponent hit a very fast loopdrive by me. I went to the wall, picked up the ball, moved back my shirt sleeve to reveal my bicep, flexed it, and gave a thumbs up to my younger opponent.