What you’re proposing is a double standard. Games cannot be moved side to side if the the intent is to move the tip of the flipper into the path of the ball, but it can be moved side to side any other time (nudging a post into the ball to save an outlane drain)? How can you possibly enforce that?
Alien Star has a post between the flippers meant to save the ball. With your policy, a player would not be able to skillfully use that post if the ball goes between the flippers...they would need to rely on luck.
As mentioned over and over in this thread, use rubber feet and a tighter tilt. Otherwise there is no way to rule fairly on the difference between a slap save and a slide save unless you’re going to get down on the floor and measure if the foot returned to its initial position or not.
Some people do believe that death saves should be allowed in competition. Despite that disagreement, the current rules are easy to enforce and do not require a judgement call.
“No slide saves” is impossible to enforce, but if you put rubber feet on your front legs and have a policy that DQs a player who causes the game to leave the feet, then you get exactly what you want. Pinburgh has done this, so there is precedent that has been accepted by tournament players and the IFPA.