With Forsaken, legendary weapons and armor in Destiny 2 will regain Random Rolls. This is likely to make most of your current gear useless with the expansion.
Forsaken brings back a feature that Destiny 1 players missed: In Year 2, legendary weapons will have random rolls again. Armor will also receive various perks, likely randomly. Additionally, there will be slots for the new mods. This is expected to make your current gear quickly lose its usefulness in the upcoming expansion.

Will your Year 1 gear become useless?
Farewell to current gear? Although Bungie announced that your arsenal from Year 1 will remain relevant in Forsaken. So far, players assume that this only refers to a few Year 1 weapons, which will also receive Random Rolls in Forsaken.
Popular Year 1 items and the reworked Exotics will still have their place in the inventories of Guardians with Forsaken, but a large portion of the accumulated weapons and armor will be useless come September.
Many weapons and armor from Year 1 will not receive any new perks based on the current information. They will still drop and can also be obtained from the upcoming collections, but will retain their Year 1 status.
Especially a shame for the Solstice armor: Players are currently particularly concerned about their hard-earned Solstice armor. This can be upgraded from a green set to a Masterwork armor through a long quest line during the “Solstice of Heroes” event.

However, there is currently no information from Bungie on how the armor will be handled in Forsaken. Guardians assume that the Solstice armor will also become obsolete with the expansion.
Players don’t want to give up their gear
Guardians are making suggestions: In the Destiny subreddit, players are already discussing ideas on how at least the appearance of the old armor sets could still find a place in Forsaken. Suggestions range from ornaments and cosmetic item slots to a transmog system.
Transmogrifications are known from World of Warcraft or Diablo 3, among others. This allows the appearance of one item to be transferred to another. This way, players could enjoy the new items with their random rolls, but still not have to give up their familiar appearance.
Are you already mourning your old items or are you happy to leave your weapons and armor behind for the new random rolls?