Hi, @jordan-violet!
In order to clear the selection in category_id dropdown, select (no category) option. This is a default selection now.
If any category was selected, you can reset selection by choosing (no category) option, which is listed first in the dropdown.
IMPORTANT!
If you want to allow query run without category_id provided, prepend param with null keyword. More detailed instructions are laid out in Using Parameters in Data Explorer Queries.
Related code changes were done in UX: category-id-input: allows no-category selection by yuriyaran · Pull Request #377 · discourse/discourse-data-explorer · GitHub.