Camille's passive shield could be a good precedent for Support-friendly Barrier replacement
[New] Adaptive Barrier
- Target allied champion receives a shield that absorbs exclusively either Magic or Physical damage the next time they are damaged by an enemy champion, based on which is most effective.
If you're feeling frisky, you can say taking True Damage increases the size and/or duration of the shield (though doesn't absorb any True damage).
Exhaust is definitely a very powerful ability, but the greatest downside for any squishy that selects it (usually Support, the squishiest of squishies) is that you actually need to get close to the enemy to use it. Exhaust would have a higher potential for reducing damage still, but Adaptive Barrier can more reliably protect one individual from one damage type without the user necessarily being near danger. I think this would be great for any solo laners in laning phase, and for Supports that don't want to get so close to the enemy.
I've thought of having a Spell Shield Summoner you can cast on an ally, but it would too easily invalidate significant purchases and picks that have it; we don't want to damage their relevance.