I mean, I'm all for the idea, but I'm not sure it solves any of the fundamental issues with shielding currently...
1.) Shield Stacking makes prime targets unkillable for large windows of time.
2.) Very little skill involved in shielding a target from a safe distance for extended windows of time. You also don't enter combat when shielding said target, even if THEY are.
3.) The 'core' enchanter shield supports can keep shields on a target indefinitely through CDR.
4.) Multiple items have enhanced the value of shields to inflated values, making getting around shields harder than ever.
Your suggestion solves point 4... but that's about it. I also think you introduce some new problems with certain combinations being very strong or being weak. If Morgana's shield didn't stop all forms of CC - she would be hot garbage. If Janna or Lulus shield only stopped physical damage, I think it would be a strong enough nerf to stop them from being a competitive or popular pick.