Per this discussion on Discord:
How might we (briefly!) annotate or note for students and/or implementers that a particular exercise has not been implemented on a track on purpose? I don’t want to create a bunch of extra work, but I also don’t want to field lots of questions if I can avoid it. 
…and for some tracks there are problematic exercises that may be implemented, but they’re not really ideal. One such for Python is Yatch, which a ton of students have solve in a very problematic way by assigning names to lambdas.