Hello
Bungie recently announced the content sunsetting with the introduction of The Witch Queen expansion and Destiny 2 Year 5 on February 22nd, 2022. Among this content is the seasonal activities from Year 4 and the weapons associated with those activities. Fortunately, every seasonal activity is still available so returning players and unlucky players have an opportunity to pick up the best items from Year 4.
These are the weapons I'd want to have going into Year 5 that have a time limit to acquire.
Season of the Hunt (Season 12)
All of these weapons are terrible. Blast Battue was interesting as the only farmable auto-loading Heavy Grenade Launcher with the ideal Adaptive Frame. This never caught on as Wendigo 2.0 as the game provided equivalent (or better) damage options that required far simpler execution. Go ahead and delete any of Crow's stuff that you're still holding on to.
Season of the Chosen (Season 13)
Extraordinary Rendition
Roll: Fluted Barrel / Tactical Mag / Outlaw / One For All
Submachine guns, as a category, are the highest DPS primary weapons in Destiny 2. Season of the Hunt's Friction Fire had a brief moment in the limelight because of this. Everyone deleted it once they got Extraordinary Rendition. Extraordinary Rendition uses one of the cleanest sights in Destiny 2 and introduced the fantastic One For All damage perk. One For All provides a 35% damage buff for 10 seconds after damaging 3 enemies. That's great for harder content such as Master-tier raids and Grandmaster-tier nightfalls where you can't rely on a constant stream of kills to keep Rampage going.
I don't think SMGs have the magazine size to justify using Subsistence, especially in harder content. It is something to consider for other content though.
Threaded Needle
Roll: Fluted Barrel / Accelerated Coils / Field Prep / Vorpal
Linear Fusion Rifles are excellent weapons because they offer a solid combination of DPS and total damage. This roll maximizes your DPS with Accelerated Coils while also maximizing your total damage with the 3 bonus reserves provided by Field Prep. Vorpal contributes to both with its 10% bonus damage. Plenty of weapons offer better DPS, but they run out of gas well before the boss runs out of health. That's why you want an LFR. Threaded Needle is the only LFR with this excellent combination of perks available.
Season of the Splicer (Season 14)
Cartesian Coordinate
Roll: Red Dot / Accelerated Coils / Feeding Frenzy / Vorpal
This isn't technically a Season of the Splicer weapon but the season did introduce the Sol System Archive engram that contains it. After the rework to Fusion Rifles, Rapid-Fire Frames such as Cartesian Coordinate became the best type of Fusion Rifle and Cartesian Coordinate is the best example of a Rapid-Fire Frame because it is the only one with Vorpal. Vorpal gives Fusion Rifles 15% bonus damage, which is more than other damage perks offer. Its main competitor is High Impact Reserves, which is approximately 7% bonus damage averaged over the magazine.
After the nerf to Slug shotguns, Rapid-Frame Fusion Rifles became the highest DPS special weapons. They'll be top-tier even without Season 15's Particle Deconstruction mod.
Chroma Rush
Roll: Arrowhead Brake / Extended Mag / Subsistence / Rampage
Add a Backup Mag mod on top of this and you'll have 70 rounds in the magazine. That means Subsistence is returning 14 rounds per kill so you can shoot forever. Rampage further enhances the best-in-archetype DPS offered by 720 RPM ARs. This makes for a very fun and effective weapon.
Roll: Smallbore / Accurized Rounds / Dynamic Sway Reduction / Tap The Trigger
It isn't worth going out of your way for this roll, but it is worth keeping if you come across it while farming for the ideal PvE roll above. 720 RPM ARs offer among the lowest time-to-kill values in the Crucible. The challenge is keeping your spray on your target long enough to achieve that. You won't find an easier way to do that than these perks.
The issue for PvP is that the best SMGs outclass 720 RPM ARs in essentially every way. If you want to spray people down in Crucible, you're probably better off choosing an SMG or a 600 RPM AR with significantly greater range such as Forward Path or Scathelock.
Ignition Code
Roll: Quick Launch / Spike Grenades / Slideshot / Vorpal
This is a kinetic breech-loading grenade launcher. For some reason, they decided to let it roll with Slideshot so you can spam grenades as quickly as you can slide. That's pretty great. It also happens to get Spike Grenades and Vorpal, which are the largest damage boosts available for grenade launchers. Don't miss out on this fun and unique weapon / perk combination.
Sojourner's Tale
Roll: Fluted Barrel / Accurized Rounds / Quick Draw / Opening Shot
This is the Energy slot equivalent of Blasphemer, which is a Slug shotgun that's dominated the Crucible since its introduction in Season 8. Being in the Energy slot, Sojourner's Tale allows you to still use The Last Word so you can double down on weapons made OP by controller's aim assistance.
Gridskipper
Roll: Arrowhead Brake / HiCal / Heating Up / Snapshot Sights
540 RPM Pulse Rifles are back in a big way in Crucible and Gridskipper is leading the pack. I've farmed every roll you can of Darkest Before, so I know that Gridskipper is the way to go. Beginning in Year 5, all Pulse Rifles will have Full Auto intrinsically, which is currently the main advantage of Darkest Before over Gridskipper. Otherwise this archetype suffers from a clunkiness caused by poor handling stats, but Gridskipper can offset that through Snapshot Sights.
Season of the Lost (Season 15)
Fractethyst
Roll: Full Choke / Accurized Rounds / Quick Draw / Opening Shot
This is the only pellet shotgun that can roll with Quick Draw and Opening Shot, which makes it the best special weapon for your kinetic slot for Crucible. Opening Shot and Full Choke combine to maximize your one hit kill range and consistency. Quick Draw allows you to respond quickly when someone tries to rush in for a melee kill or with an inferior shotgun.
How to farm these rolls
All of these weapons are available and targetable by focusing Umbral Engrams with the Prismatic Reactor located within the HELM. In exchange for various seasonal currencies, you're able to convert an Umbral Engram into a random weapon from a very limited pool of potential weapons. Season of the Lost focusing requires the Parallax Trajectory currency and Season of the Splicer focusing requires the Decrypted Data currency. Both currencies may be farmed quickly from the Expunge: Tartarus mission on Europa (or its Corrupted counterpart.)
Season of the Chosen Umbral Engrams require Hammer Charges to focus. You earn these charges by completing Battlegrounds under the Strike playlist node. That is a slow process. Fortunately, both Extraordinary Rendition and Threaded Needle are available through Season of the Lost's Tools of the Chosen engram. This uses Parallax Trajectory like the other season 15 engrams.
Destiny 2 offers many more choices of effective and interesting weapons. These are simply the ones that'll no longer be available in the relatively short time remaining in Year 4. If you're looking at all the content available from Year 4 and feeling overwhelmed, this is my answer.
