Destiny has a new mechanic where older weapons and armor can no longer be infused beyond a certain power level. You can read the details on Bungie.net.
Starting in season 11, the API DIM uses to get information about items includes the "power cap" value per-item. DIM now exposes two new search terms,
sunsetsafter: to help you identify these items. We also show the max power level and the last season an item can be max power in the item popup, the Organizer, and in Compare.
Because Garden of Salvation and Last Wish raid gear was incorrectly capped in Season 11, you can use the search
sunsetsafter:arrival -source:garden -source:lastwish to accurately highlight gear that will not have its power cap raised going into Season 12.