There is currently no perfectly reliable way to know whether or not an avatar is culled for any given user. So, if a script is tracking a remote user's hand bones for example, and the local player enables avatar distance culling set to a very short distance, all of a sudden that hand tracking gets completely thrown off. There are ways around this, kinda, but none are nearly as elegant as a simple "OnAvatarCullingStateChanged(bool isCulled)" event would be.

There could also be an "IsAvatarCulled" property on instances of VRCPlayerAPI to go along with this, or even instead of the event; either option would be sufficient on their own.