Not all "Contests". Certain events have such in their specific event rules. However at most such events I have attended, that rule is ignored, so if concerned, check with the event management prior to arriving.

And for BillyGoat, I don't think you are losing it. Back in the '80s there were so many recommendations, changing like the weather, who could stay up with stuff? Definitely not me!! AMA document 535B has this but it is simply a suggestion, a model from which clubs can modify for themselves. "FLYING SITE OPERATIONAL RULES" as a suggestion for a club: "3. Mufflers are required on all models. (db limit recommended). " Again, just something for a club to think about.