Drupal - Generate only parent taxonomy terms in a block

In Drupal 7 to only show parent terms you add a “Contextual Filter” and add “Taxonomy term: Parent term,” then set “Provide default value” to a “fixed value” of 0.

That will filter out the child terms.

You can just add a normal filter, set the "Parent term" filter to "0", that does the same as the contextual filter with "provide default value" as mentioned above. Turns out the root terms have a parent of 0 which I only just found out thanks to this post :)