Trying to strike a balance between raw enumerations of all valid states and practicality:

Rooks and Knights are identical, so their positions fall into [64 choose 2] states + 1 state for when both are captured (only one can occupy the King's location) + 3 states for when both are in the starting position and castling is available for one or the other or both

  2020 states = 11 bits * 2 colors * 2 piece types = 44 bits

Bishops only occupy half the board (32 states) + 1 state to track captures

  33 states ^ (4 unique pieces) = 21 bits

Queens and Kings just store their location

  64 states = 6 bits * 4 pieces = 24 bits

Pawn promotions uses the same method as the article

  9 bits x 2 colors = 18 bits

En passant can be stored by the column + 1 state for none

  9 states = 4 bits

Pawns can be in, uh, [64 choose 8] position states. (It's only 4 billionish)

  [64 choose 8] states = 32 bits * 2 colors = 64 bits

And captured pawns can be 'unpromoted' and placed on an empty spot in the top row since unpromoted pawns will never be there.

And 1 bit for whose turn it is

  1 bit

Total = 176 bits or 22 bytes

Started out thinking about ways to use more of the duplicate pieces, rediscovered the idea of ranking and unranking, started to understand what the person with a limit of 19.2 bytes was doing, tried out just treating position state as [64 choose 32] and only got to 194 bits, then finally worked through this approach.