Only a few Axis aircraft fall into that category though. The Ta152, He162 and perhaps the Me163. All other Axis aircraft in AH saw significant combat. The Do335 fails the criteria even though it was test flown in skies full of Allied aircraft. Conversely only a few Allied aircraft fail to get in due to it, the F7F, F8F, Spitfire F.21 and perhaps the Meteor Mk III.
There is a thread created recently asking for the only semi-decent Japanese tank, but it fails for the same reason the F8F does, they held it back and it didn't see combat before the war ended. If the nukes had not been used both that Japanese tank and the F7F and F8F would have seen combat in the invasion of Japan. Thankfully that didn't happen.