Go into TaskPaper’s application support folder… one way to do this is Window > StyleSheet > Open StyleSheet Folder, then go to that folders parent folder. Once there find the “Configurations” folder, and inside that folder you will find a “tags.taskpaper” file. Open that and you can edit “Include Tags” which always show in the sidebar even if not in your document, and “Exclude Tags” which never show in the sidebar, even if they are in your document.