About a month ago, you explained that hard CC on items runs into counterplay concerns when you can just chain it forever with your own CC. But don't heals and shields have the same problem when stacked by multiple allied champions?

An overabundance of shields and heals certainly can be oppressive as seeing all your effort and damage undone can give the player a feeling of powerlessness. The main thing stopping healing from being too overbearing that is that their direct counter (damage) is an easy mechanic to stack and can be done regardless of team composition (except maybe the rare Sona, Taric, Soraka, Nami, and Alistar comp).
In addition we tend to tune healing numbers very low even for our super healers (ex- Soraka). Each of our supports can do a decent amount of damage, even low damage supports like Taric and Sona have a good amount of burst damage, especially with a little AP, the same stat they need to amplify their healing. By making these champions damage scale with their healing it helps us keep fights moving forward and avoiding all out healfests.
Our other primary forms of mitigation, lifesteal/spellvamp and shields each have their own gating mechanisms. Shields are temporary and vamp requires you to deal damage to heal. These mechanics keep the fight from moving backwards, one team is always going to be attrition-ed down over time.
The other issue with CC stacking is just generally the long term loss of control of your champion in a fight is really frustrating and tends to lead to very unexciting fights where the receiving player doesn't feel they could have played it any better for a better result.

The answer hasn’t got any rewards yet.