Twitch.tv's Tag System Is Missing A Few
As best I can tell, the new tag system used at Twitch.tv to help discovery does not contain a tag suitable for level design streams. This is clearly an oversight, and you can give them feedback and suggest they add more tags.
The announcement is at https://blog.twitch.tv/introducing-tags-and-new-categories-33744ef7b04f
The feedback form is at https://docs.google.com/forms/d/e/1FAIpQLSd2ZNywaDrcRToquXhXr43krj0zLzyBE6pueNzME4iDY1bWLQ/viewform
The current list of tags is at https://www.dropbox.com/s/2mswa08a1wworo5/New%20Tags%20%28Public%29.xlsx?dl=0
The most relevant tags I can find with a quick scan are just the generic Game Development and Modded for playing a game "that has been modified to change visuals, add additional items or maps, etc." Neither of them are really about creating mods or specifically level design.
Consider putting in your two cents and suggesting additional tags via the form. I think Modding for general modding activities and Level Design for mapping and related activities would be good suggestions, and if you can think of others, great.