This story dates back to the days when getVariantBitmask() was introduced. The purpose of this function was to allow the variant info to be extracted from the metadata, for use with item drops. This was later changed to state bitmask for reasons I don't clearly recall.
In the great 4.0 refactor, we now store variant magic numbers separately, so we don't need any generic bitmask to split up variant and state information anymore. Variant is now only ever serialized and never deserialized. The same thing goes for blockIDs. States are read from the world by matching the full stateID against a table of prefilled known blocks, so the variant doesn't need to be deserialized - only the state does, and the state metadata readers already do bit fuckery by themselves and don't need this mask - notice how little actual changes were required to get rid of this?
This filters out over 200 invalid states which were previously considered just fine, including zero-width cakes, buttons with broken facing values, furnace/chest with crazy values, and more.
This is a major change to the way block metadata is handled within the PM core. This separates variant metadata (which really ought to be part of the ID) from state metadata, and in a couple of cases flattens separate states of blocks together.
The result of this is that invalid variants can be much more easily detected, and additionally state handling is much cleaner since meta is only needed at the serialize layer instead of throughout the code.
