Avatar SDK version 3.4.1
Reproduction Steps:
  1. Create a new Avatar project using the VCC.
  2. Select any GameObject and click "Add Component"
  3. Search for "Impostor", "VRCImpostor", or anything similar.
  4. Notice that the component doesn't show up.
Workaround:
  1. Go to "Packages/VRChat SDK - Avatars/Runtime/VRCSDK/Plugins" in the project view.
  2. Expand "VRCSDK3A"
  3. Drag and Drop "VRCImpostorSettings" onto a game object in the hierarchy or inspector
  4. Observe that you get the default unity inspector layout for the component, rather than the nice looking ones that VRChat creates.