To be honest, I don’t mind either way on that particular piece. It is a piece of etiquette that has been taught in all of the places I have played, but perhaps it isn’t a widely-shared tenet of the spirit of the game. My personal preference (perhaps due to bias from how I was taught the game) is that players on the sideline can remind teammates on the field to watch for things by saying something like “watch for double teams” before or after the fact, but in the moment it is happening, it shouldn’t be signaled/suggested in any way from the sideline. It can cause confusion because of the nature of self-officiation.
The main point I feel strongly about is that wherever the boundary is for sideline participation, I think there should be rules and accountability for that boundary.