Burning some of their tokens depends on the strategy of the team and most of them were written on their roadmap. You will be able to track those if they fulfill their promise by looking on their contract on the blockchain. Burning tokens was very effective on 2017 where the price went up a price but if you will ask most of the investors, burning tokens nowadays no longer have that impact or may take effect when the bull run starts.
developers actually have two choice with their unsold tokens.first one usually developers team will burn this token and most of investors agree with this action.and another choice was use it as airdrop for token holder.this way very effective to make investors hold their tokens,so price will be stable.