ISTM that it's important that eventually ParseVariableBool() and \if agree on what evaluates to true and false (and the more straightforward way to achieve that is by \if calling directly ParseVariableBool), but that it's not productive that we discuss /if issues relatively to the behavior of ParseVariableBool() in HEAD at the moment, as it's likely to change.
I'd like to keep in sync with ParseVariableBoolean(), but
Also, Fabien has made a good case for invalid parsed values being an ON_ERROR_STOP-able error, and not defaulted to either true or false.
This might be asking a lot, but could we make a "strict" mode for ParseVariableBool() that returns a success boolean, and have the existing ParseVariableBool() signature call that new function, and issue the "assuming " warning if the strict function failed?
