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| = ∆ID / ∆VGS. Therefore, |Yfs| can be read from the ID-VGS curve or the VGS-based ID-VDS 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.