https://github.com/click4dylan/CSGO_GameMovement_Reversed/blob/master/IGameMovement.cpp
https://github.com/ValveSoftware/source-sdk-2013/blob/master/mp/src/game/shared/gamemovement.cpp

i havent actually tested this and u will need to update the plugin with practical functionality. this is just a concept, you will need to add checks for each client per tick, updated gamedata, and update specific functions depending on the game. these functions themselves may be outdated now also, but heres the idea:

in CCSGameMovement::AirMove the player's wishspeed is calculated based on their forward, side, and up, capped sv_air_max_wishspeed (usually 30). wishspeed is then passed to CCSGameMovement::AirAccelerate, which applies air acceleration to the player's velocity based on the wishspeed, wishdir, sv_airaccelerate. the resulting acceleration is added to the velocity.

however, CCSGameMovement::ApplyDuckRatio (found in reversed igamemovement) scales down the player's forward, side, and upward movement values, as well as their maximum speed, by multiplying them with a duck ratio value that decreases from 1.0 as the player becomes more crouched. so this means, your wishspeed in CCSGameMovement::AirMove is also scaled down by this same value, and used to calculate airacceleration.

as a result of this CCSGameMovement::ApplyDuckRatio func, ducking creates a reduction in airaccel while surfing. this kinda aims to detour it and reset to 1.0 if we are airborne (we ignore if on ground). CCSGameMovement::ApplyDuckRatio is only relevant to speed reduction so this should not effect any other properties of crouching, such as player dimensions.