What is Vcc-2 referring to in the Texas Instruments Tech Sheet for the SN74AS138?
It is a simple sum.

It refers to the output voltage when an output is HIGH (\$V_{OH}\$) as a minimum (it's in the MIN column) of 2 volts below \$V_{CC}\$, so if \$V_{CC} = 5V\$ then \$V_{OH}\$ will be at least 3V.

For CMOS you will often see something like \$0.6V_{CC}\$, where the value is 60% of the \$V_{CC}\$ value.