We're still over a month away from The Witch Queen's arrival, which means there's plenty of time to go through whatever content you missed during Destiny 2's fifth year. If you're new to the game or returning from a bit of a break, Bungie has outlined everything you need to get your hands on before it gets placed in the Destiny Content Vault.
We already know that the Forsaken campaign and the Tangled Shore are going in the DSC once Witch Queen arrives, but a Reddit post by user Spawnling summarizes everything that's leaving in February using several Bungie sources.
To summarize, all the seasonal content that's been around for the past year or less will disappear with The Witch Queen. This includes much of the H.E.L.M.'s inhabitants, including the Eliksni and Awoken wings, Mara Sov's Chambers, the Splicer Servitor, the Wayfinder's Compass, and the Umbral Decoder. Note that this means you'll need to grab any Elemental Well mods now before these sources get vaulted.
Spawnling notes that while there's a long list of items that are getting Vaulted with Witch Queen, Umbral Engrams aren't among them. This likely means that the Prismatic Recaster and Umbral Decoder will either be relocated or perhaps integrated into the coming weapon crafting system.
With the Presage and Harbinger missions becoming vaulted, Guardians will no longer be able to obtain either Hawkmoon or Dead Man's Tale. Several Exotic Ghost Shells, Ships, and one Sparrow will also be unobtainable as they're associated with Triumphs that can only be completed in the soon-to-be vaulted missions. Best get those while you can.
With the Tangled Shore disappearing, several Exotic Weapons will have their associated quests removed. Wishender, Lord of Wolves, and Ager's Scepter will only be obtainable from the Lost Lights Exotic Archive, while the Chaperone can still be obtained via a new quest from Shaw Han.
One of the better sources of high-stat armor will leave with the Umbral Recaster. Season of the Splicer had a third-tier focusing option that produced targeted, high-stat armor that can be focused further using Ghost armorer mods. If you need some good armor, be sure to farm Splicer materials before they're deprecated.
Lastly, while Bungie didn’t write down what might be changing to weapon sources come Witch Queen, the new expansion will almost certainly result in some weapons being removed from their loot pools. Best grind out whatever you’re missing now before February 22.