(yes, I know Jax's E blocks AA already, but Shen's a tad unique since it blocks for both him & his allies)
Why not a kit which eliminates existing shields like Janna's E & Nautilus's W? That's another interesting kit you guys could imply for future new/reworked champions.
I'm sure it's something they've considered in the past, and it would be unique, but I think it would actually be a very limiting mechanic.
Think about it. Being able to eliminate shields is extremely powerful, and many champions rely on their shields to be useful, and I don't even just mean Supports. Think of how much 
and 
and 
rely on their shields not only for protection, but for their offensive presence. Such a champion wouldn't just get rid of giant shields on ADCs, but offensive shields on tanks.
In such a world, squishy, fast champions would become the norm. Yasuo becomes even more of a dominant mid-laner since Orianna, Lulu, Morgana, Karma, and Lux would become obsolete. Such a champion would easily attract bans just so champions with shields could be played.
I'm not saying it's impossible, but it'd need to be done carefully. There's a lot of ways to mess it up.
Say it's a powerful ultimate that removes all shields around the champion, akin to Sombra from Overwatch. Such a champion would become a rather niche pick, since they'd have no ultimate VS a team of champions with no shields (Something like 
). Yeah, it could remove BT and Locket shields, but is that really worth an entire ultimate?
If it's a basic ability on a champion's kit, no way could it be an AoE ability. No way could it even remove a shield completely, since it would still invalidate/hard counter tank/fighter champions who rely on their shields. It could be an ability that simply does increased physical/magic damage to shields, so it could be itemized against, but that might end up just being too niche also and it would just be a nifty keychain tied onto an ability. Again, it'd need it's other strengths buffed so that it could even play against champions or teams without shields.
I think it's a really complicated idea. It's possible, but it can be done wrong so easily that it needs to be handled with precision and care.
It would be way too situational, and against champions or comps that doesnt rely on shields, it would be useless and that champions who got the mechanic would only have reason to be picked in countermatchups
