Another way to look at it is that any runway where the perf data says you'll need more than 60% of it is "short". Works at any WAT too!
One charter company I worked for had authority (EOD operator) to operate to airports where up to 80% of the ALD could be planned to be required. Hence, these runways were considered "short". Certain operational restrictions applied of course.
With all the mountain resort destinations we flew to in the small jet charter biz, I also came to consider any runway with an elevation greater than it's ALD to be one worthy of extra respect, particularly in winter weather. Common destinations like Telluride, Co (9,038 elevation and 6800' or so ALD) stand out as an example.
One company that merged with ours used an arbitrary 5,000' to define a runway as being "short". I always considered that viewpoint to be rather "short-sighted". The pilots from that outfit adjusted to and came to appreciate our approach to the matter of defining a short runway.
I understand that airlines operating larger airplanes may have a different take on it, but I thought the above to be worthy of consideration.
Best,
westhawk