On the newest Open Beta, Physbones that are children of other bones affected by Physbone components jitter horribly, even if the child bones are marked as excluded in the parent's Physbone component. The first gif attached is how this looks on live, and the second gif shows how Physbones that did not jitter before now do so on the beta branch.
This avatar uses several Physbone parameters to drive animations keeping the lowerbody of the avatar upright. Since one of these Physbones is located on the hind hips, the tail is a child of that Physbone. Even though it is excluded in the hind hips Physbone, notice how the tail still jitters horribly in the beta.
The avatar featured in the gifs can be tested with this link: https://vrchat.com/home/avatar/avtr_edde17d2-800a-419b-ad2a-afa244a4eb24