It is currently possible to view all the items that have one or multiple tags by clicking on them in the Tag Selector. I would like to make a "negative" tag selection, which is to be able to view all the items that do not have a specific tag.

This has been suggested before:
https://www.zotero.org/trac/ticket/658
https://forums.zotero.org/discussion/853/tags-what-could-be-improved
https://forums.zotero.org/discussion/73769/feature-request-ability-to-view-citations-which-do-not-have-a-specific-tag
https://forums.zotero.org/discussion/74795/new-feature-request-3-states-for-tag-selection
https://forums.zotero.org/discussion/85871/exclude-tag-from-selection

Instead of the solutions proposed there, I think that adding a "+/-" toggle button at the top right of the Tag Selector (or equivalent UI) would be more intuitive than using a modifier key or multiple clicks on the tags.
1) The current positive selection "+" shows a blue background on the selected tag. The negative selection "-" could simply show a red background on the tag.
2) The same background colour could show on the "+/-" toggle button to make it easy to understand visually.
3) This may be hidden to save space, and be accessible from a pane top-right arrow to expand the toggle button, or even from the bottom-right menu.

I understand that you can use an Advanced Search to do that in most cases, which is however much slower. But this solution does not work in the Duplicate Items collection:
https://github.com/zotero/zotero/issues/1087
This would be very useful to hide the "Non-duplicates" tagged items from the Duplicates Items collection:
https://forums.zotero.org/discussion/100980/select-merged-item-after-merging#latest

Note that this feature with a toggle button could then be expanded to a "And/Or" toggle button, equivalent to the "Match all/any" option in the Advanced Search.
It could then be expanded to the selection of multiple collections (within a single library):
https://github.com/zotero/zotero/issues/177