Bungie announced the upcoming changes to Legendary weapons and armour which will make anything you grind for useful for up to one year, maximum. Needless to say, this didn't sit well with the community.
This Week At Bungie posts sometimes serve as the developers' ace in the hole and sometimes as massive disappointments to the community. On May 14, 2020, the latter happened.
Bungie discussed ways to keep things fresh regarding the gear farm in Destiny 2, noting that "if a new Legendary weapon is better than the current best-in-class, [they] risk power creep, removing challenge from the game, or making the item mandatory/the only option for challenging activities". To that end, they decided to make Legendary weapons and armour from seasons 1-8 obsolete, come Season 12.
This will happen because the items from those seasons will be power capped at 1060, which will be the maximum that can be attained in Season 11. For some regular activities, like Strikes and normalised Crucible, this will not matter but those looking to try Iron Banner or Trials - we've got bad news for you.
Moving forward, all the Legendaries will behave this way as they will be infusable up to the power level of the three seasons from the one where they were introduced. In other words, each Legendary weapon and armour piece will be relevant in up to four seasons, or one year from the moment of their introduction.
Considering that no one likes farming in order to masterwork their armour, putting a soft expiration date on it is bound to make players even more disillusioned.
Transmogrification will help players retain their armour looks as the older pieces become obsolete but it remains to be seen what Bungie will do in order to mitigate the damage to those who bought cosmetics for weapons from older seasons.