NOT IN MY GYM – On match point for the goal the suggestion of push ups is just not relevant.
I think penalties given to players by coaches is a just trying to show the coach is tough.
Have I ever given penalties? Yes. If kids take too long to get in after a whistle maybe the last kids gets 10.
If a kid hits another kids in the face with a ball when picking up all the balls maybe.
I would always make it clear why they are doing the penalty and make it fun.
As a coach I would often do the penalties with them to show we are a team.

Doing penalties for missing a serve is (in my opinion) JUST PLAIN WRONG. You are better off teaching the kid to serve. If they are not concentrating enough, teach them to concentrate better. If they are just not motivated enough, suggest they play another sport.
WALL SITS – I hate them. Unless they are part of a 3 times per week training program to help players stay in a low position in defence then, they are just an ego trip for the coach to show they can inflict pain on their players or they are just so tough. When I teach defence, I teach them to be low and mobile not stationary against a wall!