I like to think valve would have a better way of telling which are popular than "how often is it played."
"How often a map is played" seems like a pretty important metric for determining popularity. You just can't use that to exclusion of all else.
Dustbowl was THE most popular map by a huge margin when community servers made the majority of available games... but it was also the first map in the rotation so a server sitting with a single spectator for source mod would sit on dustbowl forever.
On the flip side, stuff like arena, tc and pd aren't supported properly by Valve servers now, so ANY map with those prefixes are forced to be unpopular because the only way to play those maps is to join a (comparatively rare) community game.
Ultimately metrics are up to Valve since they have more info than we do (and we can only guess as to what they track). Logically you'd think it's stuff like this:
* Frequency of full servers with this map
* Total time servers spend on this map
* Frequency of map extension votes passing for the map
* Frequency of votes passing to switch to this map
* Frequency of connections/disconnections when on this map
* Map stamps & map filter sales
(That said, I don't think stamps or filters should account for a huge factor in any case, since those sales look pretty pathetic compared to the campaign pass sales -- but improving the value of stamps & filters is another conversation entirely so I digress)
I'm guessing Valve-made content is not up for removal since we're talking about using workshop entries to provide the content. Which is too bad, because I wouldn't be opposed to
removing the oldest or less exciting arena maps.
Halloween maps seem like a good candidate by their seasonal nature. Most of the year, you never even load them up, but if you download the game in April, you're downloading a ton of Halloween maps that you wouldn't see for months. I feel dirty even mentioning this because we just had a huge quality community Halloween update last October.
Someone already mentioned how
the HL2 content which goes mostly unused in TF2 could be culled. Most of it probably could (some maps do use things like stair textures or aerial props I've noticed, but there's no reason to keep Combine stuff, is there?)
I don't feel it's fair to remove community made maps for gamemodes that aren't played on Valve servers though. Don't have a solution for this... Having a set of Valve servers with a hodgepodge of gamemodes doesn't sound like that good an idea either. Are people going to want to go through a player destruction game followed by an arena game and then a medieval mode one? Maybe PD could be combined with RD once asteroid is done since the gamemodes are similar enough.
I'm not opposed to removing maps... but I think picking older and unpopular maps as well as culling unused content should be done first.