Workshop is a lost cause, just use l4dmaps[www.gamemaps.com]

Use https://www.gamemaps.com/l4d2/campaign/
And then, if you want, search for the campaign's title on the Workshop.

For some reason, Demo mentioned "l4dmaps", although it has been changed years ago and sent the l4d1 link instead of l4d2 lol

The subtract option is mandatory. Try this, it doesn't block out everything that isn't a campaign but the list should be nearly all campaigns and maps: Filtered Workshop

Thank you, last 2 answers are quite good.

It's easier to find custom maps on the GameMaps website. The Workshop shows a lot of custom maps and mods all mixed together. Unfortunately, some custom maps can only be found in the Steam Workshop.