Years ago, Bungie decided that a necessary step forward for Destiny 2, its sandbox, and player habits was to put a firm cap on gear. This became known as sunsetting and essentially it saw gear reach a power cap around a year after it was originally acquired. Fans didn't exactly like it as it meant some previously very hard to acquire loot was basically useless after a certain time period, but no one can deny that it made the meta of Destiny 2 much less repetitive.
However, now that we're almost at the end of the road of the Light and Darkness Saga, Bungie has announced a few methods to bring back and ensure that all Guardians, new and old, are able to join in on the effort and to help defeat the Witness. This includes a new Fireteam Power system where activities scale to the highest-power player in a group, with the rest of the group upscaled to that point so anyone anywhere can play together. This is on top of a lower soft power cap than we've seen in the past (increasing by 40 and not the usual 150), and also the sunsetting of sunsetting.
Yep, all of those beloved and older pieces of loot you spent years farming for are usable once again. Bungie has explained its thought process on this matter in the latest blog post, where it states:
"Always being able to play with your friends was a huge goal for us. As we close out the Light and Darkness saga, we want to rally all Guardians (active, returning, and new) to help fight the Witness, and there is no better way to play Destiny than with your fireteam. As we pursued this goal, it became apparent that Power limits were fundamentally incompatible with Fireteam Power."
If you have dismantled any former items to make space in your vault, tough luck, there's no getting them back. If not, you can pull them out, dust them off, and keep using them like it's 2020 again. Granted, any items that have already been introduced will feature access to the new traits and perk systems, and are therefore likely much better, but that doesn't change the fact that you can use some of your older weapons once again if you so please.