Thanks for this thread!
Name of the item: Secret Agent
Category of the item: Outfit
How was it previously released: Talus Engineer Corps Dungeon Quest
When was it last released? Last obtainable in 2018 before the removal of Viridian-Moonwater dailies.
* This one was mentioned previously and I'm seconding it. I farmed for this outfit endlessly back in the day, but my RNG was never good enough to get it before the quest was removed.
Name of the item: Purity
Category of the item: Outfit
How was it previously released: 2nd Anniversary event
When was it last released? January 2018
Name of the item: Blue Sunglasses
Category of the item: Face Adornment
How was it previously released: It's listed under "Hongmoon Store" in the Wardrobe, so I assume it was released there at some point.
When was it last released? I have no idea as I've never seen it personally and could not find any information online.
Name of the item: Starslayer, Starslayer Cape, Starslayer Hood, Starslayer Mask
Category of the item: Outfit, Adornment, Head Adornment, Face Adornment
How was it previously released: HM store box
When was it last released? February 2018
Name of the item: Shadow Crusade, Shadow Crusade Hood
Category of the item: Outfit, Head Adornment
How was it previously released: You could register to redeem it when FM third spec was released
When was it last released? June 2020
Name of the item: Escort Officer, Oath Token, Officer's Sedge Hat
Category of the item: Outfit, Head Adornment, Head Adornment
How was it previously released: Merchant of Wonders
When was it last released? Before the removal of Merchant of Wonders, I'm not sure when he was last available
* Escort Officer set isn't my top preference of outfit, but I never got to complete the collection on all the characters I wanted to before the merchant was removed and I'd always hoped for another way.