In WRITING-5356 we implemented support for legacy $text expression. The way it's implemented, we actually don’t use the match expression itself to evaluate the search predicate, but rather do it in a PlanStage layer, so this expression shouldn't ever appear as a filter in a PlanStage tree, meaning that we can never reach this line (and this is the reason we're not failing with an 'unsupported expression' error.

We need to change this line to MONGO_UREACHABLE along with a comment describing why we never need to translate this expression.

Same here.