I often read and heard about burning tokens that were not sold when the ICO project was finished, I also knew that by burning the rest of the token was a strategy so the price of the token didn't fall, but if the DEV and the team didn't burn it was a crime? is there a requirement that the remaining unsold tokens must be burned?
yes, and that is indeed a common strategy for a project, for reasons of increasing demand because goods are scarce and prices are increasingly expensive, and that is for projects that are running and developing, not for dead projects. there is no general requirement to burn tokens, it is only to make investors who join have hope that prices will rise sometime and there was no massive dump carried out by their team from unsold tokens.