In The Burning Crusade it is item level 141 and requires level 70. In Wrath of the Lich King it is item level 280 and requires level 80. In MoP Classic it is item level 141 and requires level 70.
In the Appearances collection, Archbishop's Slippers is listed as Boss Drop. Appearance ID: 7999. No other item uses this appearance, so it is unique to Archbishop's Slippers.
Read from the World of Warcraft client build files.
Where Archbishop's Slippers Comes From
Archbishop's Slippers Drops From
The in-game Adventure Guide lists Archbishop's Slippers in the loot of 1 encounter.
Read from the World of Warcraft client build files (JournalEncounterItem, JournalEncounterCreature, Achievement) and the Blizzard Game Data quest API.
How Archbishop's Slippers Compares
Who It Suits
Cloth is the armor type Mage, Priest and Warlock wear. With Intellect, Archbishop's Slippers suits Discipline, Holy and Shadow Priest; Arcane, Fire and Frost Mage; Affliction, Demonology and Destruction Warlock.
Transmog Sets
Archbishop's Slippers is part of the transmog set Absolution Regalia (Priest only). The other 16 pieces:
Blizzard numbers items as they are created, so the named items next to ID 30885 (30880 to 30890) were usually added in the same batch as Archbishop's Slippers: the same quest line, zone, vendor or reward set.