Movie ratings are not mandatory, at least not in free countries. MPAA ratings like “R” and “PG” are a voluntary classification system and films are free to opt out, though many theater chains may be less likely to show your film. But small theaters and streaming platforms don’t usually care.

Authoritarian states like China and the UK do require classification/certification of films before release. Imagine requiring a painter to have their paintings reviewed by the state before exhibition!