If Riot wants a mechanic that counters shielding so bad, I have a suggestion

Add a new type of status effect. I don't know what to call it but I can describe how it would work. Basically:

This unit cannot be targeted by allied effects

Current self-shielding champions are problematic for reasons that their shields are exacerbating, not causing.

• Riven is bullshit because she is loaded up with healing from shit like Conqueror and Death's Dance when she was literally designed around having low regen and poor synegy with healing items to counterbalance how hard her kit makes her to hurt.
• Tahm was bullshit because he was a fatass ball of stats that can't reasonably be outplayed in lane

That means the problematic cases of shield spam on champions are cohabitating with a problem that anti-shield does nothing to address outside of shields: piling on an overwhelming amount of protective measures to halt aggression. You can CC a champion, but that does nothing to stop said champion from having fucktons of buffs piled on top of them followed by a Mikael's, Tahm devourer, Kalista ult, or Soraka ult. A type of debuff that disables the ability for allies to target them would be far more ideal that a "delete your shield, lol" button in a case like this.