Stoneborn being pure health scaling is bothersome, IMO

Disregarding the rather measly health gain from the 5% itself, I think the issue of the scaling is twofold; that solo laners use it better, and that it doesn't solve the feast/famine problem tank supports constantly suffer from.

support tanks usually don't itemize heavily into health, or at least they don't end up with as much as solo lane tanks and juggernauts will. On my usual Taric build I'll cap out at just over 3000 health, with an equal amount of gold distributed to resists.

this is also considering the fact that with solo lane or jungle gold, if I were to play Olaf or Sejuani, I would be able to buy more health to make it scale better, and I'd have more money to have it sooner. This isn't even considering how you don't get to enjoy the sustain yourself; tank supports usually lack the ability to heal up, and the~~ one~~ two that do have a heal have a shitty heal , so it really doesn't seem as valuable as COTC is... ever, except for perhaps Tahm Kench. So, if I take it as a tank support, I either win lane and get a lot of HP/we get fed, at which point Stoneborn just becomes a win-more, sustain-more thing, or I lose lane and have to passively accrue gold so that my team sustain can eventually become meaningful.

So with the exception of TK, health-stacking bruisers and tanks can apply it more often and get greater value out of it both for themselves and their team. I don't know if it's really an issue, I'm not sure if Stoneborn is actually making tank supports good again, but you could consider either giving it some bonus resist scalings to be more suitable to tank supports and tanks, or you could make the base value scale up per your level, or per your ally's level, so that tank supports can still enjoy the ability to provide some meaningful sustain to their teammates if they've fallen behind in levels and/or gold.