I do want to acknowledge a potential alternative that doesn't make sense without a little context, but: Max HP down debuffs (iirc Suppuration is what it's called?) would also work to a limited degree. The main thing is they don't affect raw healing throughput in any way, but they do affect mitigation. Needing extra mitigation to deal with Vulnerability stacks is a common thing, but that's only if you're getting oneshot. Vulnerability (and Infirmity, for that matter) also requires you to pay attention to sustained damage sources (I know there's not many, but not every avoidable mechanic applies Vulnerability either), whereas HP Down doesn't (unless you took so many you're getting oneshot by things you're clearly not supposed to. Something that Tanks will screw up too). And given non-healer roles have a surprising amount of surplus in this area, they can help save themselves. The difference between that and raw healing is some healers (Mostly WHM, if we're being honest) don't have the tools to mitigate everything, so this puts the load specifically on non-healers in certain compositions, or at least keeps Shields relevant.
Having that be the penalty instead of damage could be an option in some cases, such as P2S's acidic water. I don't mind P2S's water as is, but some rando eating a DoT feels like far more work than having to baby them extra at a specific raidwide window. Vulnerability ranges in-between and sometimes on one of those extremes and that's cause it depends a lot more on the context of the fight.