Java Edition:Sponge
Sponges used to be unobtainable but were available in six different version ranges. Now they are obtainable in ocean monuments and from elder guardians making this a recontinued feature.
Obtaining
Classic Multiplayer Test
From sponge's addition in Classic 0.0.19a until Classic 0.0.23_01a sponges could be placed from the Creative inventory selection. To select the player had to press B to get the "Select block" menu and then choose the sponge on the second row. This version range ends with Classic 0.24_SURVIVAL_TEST which changed the player from creative to survival mode. It also had the feature of removing water source blocks in a 5x5 block range.
Late Classic Creative
From Classic 0.28a to Classic 0.30a Creative the player was in creative (Late Classic Creative). The player could select sponges by pressing B and going to the "Select block" menu then choosing the sponge on the second row. This version range ends with Classic 0.30a Survival, a counterpart of Classic 0.30a Creative that put the player into survival mode.
The sponge would later be found in the Creative inventory from Indev 0.31 20091223-1 to Indev 0.31 20091223-2 and likely in the missing Indev 0.31 20100113 (Creative) version. However, none of these versions have the ability to save and so cannot be used to obtain sponges as an item.
Indev Item Chests
From Indev 20100124-1 to Indev 20100128 new worlds would generate with the Indev House, and inside the house were debug chests. Between Indev 20100124 and Indev 20100125 the chests held items in stacks of 99, while in Indev 20100128 the chests held items in stacks of 100. Sponges were one of the such items that generated inside the debug chests.
Beta 1.6 Test Build 3 Debug Chests
In Beta 1.6 Test Build 3 new worlds would generate with 11 debug chests. Within the debug chests were a wide variety of items most of which were available in there full stacks some were not. Sponges were one of the such items that generated inside the debug chests, sponges generated in the 1st and 5th chests from the left in the 19th and 3rd slots respectively, in stacks of 64.
1st chest from the left.
4th chest from the left.
B Creative
In the version 11w49a (1.1 Snapshot), pressing B would toggle the player between Survival and Creative mode. This allowed the player to access the Creative inventory where the player could then grab Sponges.
Structures
In 15w25a (1.8 snapshot), wet sponges began generating in Ocean Monuments and were added to the elder guardian's loot table. This recontinued sponges until 24w20a (present), but a bug in 21w13a (1.17 Snapshot) some Ocean Monuments would generate empty (without elder guardians or wet sponges). [1]