Reading the ECMAScript TC39 pipe-operator proposal; it mentions "syntax tax" when comparing 2 possible alternatives for the pipe-operator implementation (see #hack-pipes-favor-more-common-expressions).

I interpret this to mean that for example extra syntax is required. E.g. for F#-pipes you need to wrap non-unary functions in a lambda taking a single argument to work. So the "tax" is the added extra syntax that is needed to make that work.

(Perhaps this is more a question of general use of English, and I'm not used to how "tax" is used in other contexts.)