StormRel: I think I can provide a little info here.

A good example is "america burgershop vketchan", a model I built with a worldspace constraint "burger" prop. It used to sit nice and solid at the root without a constraint, but now it "wobbles" without any on my end changes to the model between those updates. I think this is what they're referring to, I noticed the changed behavior around June too. The "root" of the player isn't "solid". Might have something to do with the skeletal model or IK changes though I'm unsure, that's just a guess. Around the same time, I noticed, (very rarely), that you could sometimes appear in an unusual place instead of spawn on worldjoins too or teleports too, especially if the player is being moved. Maybe some kind of floating point precision issue, happens more often in 6pt+ tracking.

There's quite a lot legacy models that use the same idea of having a gameobject on root (not child of armature), curiously this doesn't affect all models I have where I've built a worldspace gimmick prop, but this is likely due to more aggressive use of constraints on my part.

I'm unsure if it "needs a fix" but more like "a this is why this happened", with some explainer to "use this bone instead of the root" or something. It's probably unity specific, but that's just a guess too.

you can try burger vketchan here to see the prop "wobble": just drop her burger from the expressions menu and walk around a bit.