Unavailable items were temporarily available during a seasonal event. They might come back.
World Drops are items that drop from any suitable Loot Source in addition to their other sources.
Suitable Loot Sources:
Datamined items are not obtainable with legitimate methods.
They can only be obtained using bugs or the Cheat Engine on the PC.
Quest Reward Items in Borderlands 2 can be farmed on PC by Read-Only farming.
How to Read-Only farm:
Tip: Store the items in Claptrap's Secret Stash.
This way you can get the quest reward items multiple times since they are saved on a different SaveFile.
In Borderlands 3 Quest Reward Items are rewarded after or during a quest. After the quest is completed there are 2 ways to farm base game items:
Dedicated drops are items that can only be farmed at one or rarely more sources that have the item assigned to them.
External Source items are mailed to you for activities or purchases outside of the game.