The new beta version of the IK 2.0 introduced shoulder tracking when using elbow trackers on your upper arms
However, using elbow trackers on your forearms would break your avatar's shoulders.
While this wouldn't be a problem for people using Vive or Tundra trackers, people using SlimeVR would suffer greatly from this.
SlimeVR (a cheap FBT solution using IMUs) estimates your elbows positions from your controllers and a single tracker on your forearm. The tracker
must
be on the user's forearm (I would know, I developed it :p). This would effectively make SlimeVR elbow tracking unusable in VRChat.
A simple solution for this would be to add a toggle somewhere to switch between "Elbows-only" and "Elbows and Shoulders" tracking mode.