Now that makes sense.
T20, IPL, Test matches or the Ashes, discuss it here.
The rules for valid bowling of the ball is that the arm should be above the shoulder (https://www.lords.org/mcc/the-laws-of-cricket/no-ball).
Probably the most well known and most successful to have a sideways action was Lasith Malinga of Sri Lanka (https://www.youtube.com/watch?v=nclPaVQmCHc). His name is pronounced with the "g" but the "g" was softened when some referred to him as "Malinga the Slinger"! His action was checked and approved. The reason why not many bowlers use a sideways action is that the timing of the release of the ball is super critical for the placing of the ball to the batter.