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?
There is no requirement by anyone or any law that puts them under compulsion to burn, but if they earlier announced they would burn, then they have to burn.
If they do not comply with the roadmap or agreement made, then they can be seen as scammers.
If token burn is also done, you would notice in the total supply. Also an evidence can be shown, to show the process of the burn.