Penn commented below:
Pretty sure it procs on blastcone thing (cotc did). Its not a good reason to really take it, but its usable.
^ It does proc it.
Like, it's possible we could say "this is way too niche a case" and add in the swapping protection for this one but for our first rule set around swapping we wanted to have a pretty hard line in the sand of "there is no way you could use it ever in any situation", with the examples again being Cass with Boots or ARAM with Riverwalking.
I can bring this example up with the team to make a call on if "can be used with blast cone" is basically experientially equivalent to "can't be used" (because it's not possible to use it in the base kit or through the item system, etc.), but will re-iterate the reason it's not in there right now is that we wanted the rules around the protection swapping to be super clear.
EDIT: Okay, spoke with relevant folks & we're going to add the swapping protection for Aftershock in all modes. :) Thanks for bringing this up!
EDIT EDIT: Going to actually do this work for ANY champion who can't meet the requirement in this case, not just Garen. :D
It's also possible that we further adjust these swaps in the future, wanna put that out there as well.