Are there any reasons why forward transfer admittance, |Yfs|, is not specified in datasheets?

Also called transconductance (gm), |Yfs| is the ratio of the drain current variation at the output to the gate voltage variation at the input and is defined as |Yfs| = ∆I_D / ∆V_GS. Therefore, |Yfs| can be read from the I_D-V_GS curve or the V_GS-based I_D-V_DS curve shown in datasheets.
You should read a |Yfs| value under actual usage conditions from these performance curves rather than using a |Yfs| value under the predefined conditions for the Electrical Characteristics table. That is why |Yfs| is not specified in datasheets.