Java Edition removed blocks/Metadata variants/Recontinued

Blocks resulting from extreme data values
The majority of these blocks were likely never actually meant to exist in-game, and occurred only due to the game handling extreme metadata values as it would those in usually attainable ranges, resulting in strange blocks with traits arguably analogous to garbage data.

Leaves with data value 3, 7, 11, 15
From the introduction of leaves in Beta 1.2 onward to the Flattening of 1.13, leaves used numerical data values within the leaves block to hold their type. As only three species existed from Beta 1.2 to release 1.1, with data values 0/1/2 (and by extension 4/5/6, 8/9/10 and 12/13/14), a data value of 3, 7, 11 or 15 would produce an undefined leaf type. This block appeared identical to conventional oak leaves, but appeared to use the spruce tint.

These were substituted out of the game with the first 1.2 release snapshot, as jungle leaves from then on occupied that space in the leaves block.

Seamless smooth stone slabs
When new slab types were introduced in Beta 1.3, slabs with data values up to 7 that exceeded the value of the highest legitimately obtainable slab would use top on all faces.

Two of these were replaced by the brick and stone brick slabs in Beta 1.8 Pre-release, one was replaced by nether brick slabs in snapshot 12w49a for 1.4.6, and the last remaining one was replaced by the quartz slab in 1.5 snapshot 13w02a. The smooth stone block was effectively reimplemented later in 1.5 in snapshot 13w04a by making the upper eight stone slabs use the top texture on all sides (also introducing smooth sandstone and quartz as a result), but the slab version are no longer in the game.

Strange levers
These also had weird distorted models, and could be obtained by loading a world from 1.3 or later containing ceiling levers (0, 7, 8, 15) in 1.2.5.