Destiny 2's new Power system will rotate Legendary weapons and armor on a four-season basis.
Following up on earlier comments from director Luke Smith, the Destiny 2 team at large explained the game's upcoming Power changes in its latest blog post. Simply put, Legendary weapons and armor will all have a Power limit as of Season 12 at the start of Year 4. This Power limit will be set at the player Power cap for the next three Seasons after its release, giving each piece of gear four Seasons (or one year) of limelight total. Once those four Seasons are up, that Legendary gear will officially fall behind the new player Power cap, at which players will need to obtain new gear to hit the new cap.
Gear from Seasons 1 - 8 will have a Power limit of 1010, while gear from Season 9 - 12 "will all be infusible to new player Power caps for one year after their release." In other words, the gear you got in the Season of Dawn has about seven months left, and the gear you just earned in the Season of the Worthy has about 10 months left. As a side note, the images in Bungie's post suggest Season 11 will raise the Power cap to 1060.
Note that old gear isn't outright vanishing, nor will it be unusable. It just won't be able to hit max Power, which will only really matter in pinnacle activities. And thanks to the true armor transmog system which is coming in Year 4, you'll always be able to keep the looks of old armor. Additionally, Bungie is planning a new Seasonal mod slot for Season 12 and beyond that will accept all mods introduced throughout a full year, streamlining the current system where armor mod slots just hop-skotch between three adjacent seasons.
This rotation will apply to almost all Legendary weapons and armor, but Exotics will not have a Power limit, and Bungie says weapons and armor from the Last Wish and Garden of Salvation raids will also "be granted exceptions and will have a higher max Power level." It's unclear to what degree raid gear will be exempted, but I would expect it to fall off at some point in Year 4 so that the next raid can take center stage.
Bungie says it will never directly update the Power limits of old gear, but it may re-issue old gear with a new max Power level throughout future Seasons. Here's hoping Bungie puts far greater emphasis on creating new and exciting gear rather than re-releasing old stuff, because ever since this Power rotation was first announced, players have worried that they'll just wind up re-grinding the same old gear. Fortunately, it sounds like Bungie is planning to do cool stuff with the design space this system will free up.
"The introduction of Max Power Level will allow us to create exciting new gear more frequently without significant concerns of permanent power creep, which is unsustainable in any game," Bungie says. "This also should open up creative and experimental builds, season over season, while letting you anticipate and prepare for how the sandbox will change over time."
Massive loot changes are also coming to Destiny 2, including Adept Weapons for Trials, playlist exclusive loot, pinnacle cosmetics, and more.