Transfer payments and IS multiplier
Notice that you only  consume out of your disposable income, that is, adding transfers and removing taxes. Thus, from that formula of the multiplier, yes, it should be there, in the $(1-t)$.

As (assuming) $C=cY^D=c(1-t)Y$ and thus, $MPC=\partial C/\partial Y = c (1-t)$, then $Y-Y^D=tY=\text{Taxes}-\text{Transfers}$, with $t$ "condensing" all the information.