In newer versions of Unity they have developed a new package that is integrated into the engine called Animation Rigging, which adds new types of constraints and gives a lot more control in regards to creating animations and better deformation correction tools. It utilizes the Unity Jobs system so it's multi threading compatible so it could also potentially work on Quest too. Overall it would be a net benefit for avatar and world creators since this is a part of the package manager as of Unity 2020. Here's a look at the plugin here: https://docs.unity3d.com/Packages/com.unity.animation.rigging@1.3/manual/index.html

Edit 1: This is not supposed to replace Final IK, it's supposed to be something to add to the current way most of us do our constraints and hopefully could also help in regards to avatar optimization

Edit 2: What makes this different from regular constraints is that it adds multithreading support and adds new types of constraints that would be beneficial for things like twist bones or dampening effects. Instead of supporting the add on it may be better to implement it directly into the SDK to reduce the amount of confusion for world and avatar creators. And if implemented correctly, this can reduce the CPU overhead the current constraint system has on avatars and even as I mentioned it could mean constraint support for Quest.