beside about trading volume usually exchanges team look at project developtment progress. If after several months or year there is no important developtment exchanges will warn dev team first before decided to delist them. If any die project token still in market it could harmed investors , so its good prevention and protection step from exchanges to their users.
Actually the effect of decreasing trading volume is because the impact of project development has decreased so the community has been wary of selling before the project dies, I think exchanges should implement new regulations to open reports on projects that are not eligible to be traded for immediate delisting because it will harm investors and traders.