Tags that come from file imports aren't considered automatic tags, because most of the data exchange formats don't distinguish between different types of tags/keywords.

The Zutilo plugin can remove all tags from selected items. (A future version of Zotero will include that capability as a built-in feature.)

https://www.zotero.org/support/collections_and_tags#removing_tags_from_items

You can also hold Cmd (Mac) or Shift (Windows/Linux) while dragging items to a tag in the tag selector in the bottom left to remove the tag from all of those items.

You can also right-click a tag in the tag selector to delete it completely.

This remains the single biggest issue with Zotero after well over a decade: neither Zotero nor any of the existing Zotero add-ons can batch-delete multiple tags from a library. This issue is a frequent problem when migrating to Zotero from other citation managers. Just to make clear, this is not about removing all tags from multiple entries, it's simply about the ability to select multiple tags in the Tag Selector and delete them completely from the entire library. Users may wish to keep certain tags but delete others. Large libraries will come with thousands of tags; deleting them manually isn't feasible.

I think I've worked out how to remove all tags from your library by combining advice from various threads without needing to use code;

1. If you have tags you want to keep, create a new collection (e.g. I named mine 'Saving coloured tags' < create a new folder for each tag you want to keep < copy all references with that tag into their specific folder.

2. Install the Zutilo plugin; https://github.com/wshanks/Zutilo (full installation instructions are on the webpage if you scroll down)

3. As is_PengLu has said at https://forums.zotero.org/discussion/comment/406821#Comment_406821 , "After installation of Zutilo, go to Tools < Zutilo Preference < User Interface < Remove tags [second option down] --> deselect 'hide' and select 'Zutilo context menu' so that 'Remove tags' will display when you right click an item

4. OPTIONAL: on the zutilo preferences menu (Tools < Zutilo preferences) click on the 'Shortcuts' tab on the top, scroll down to find 'Remove tags', select it, and enter a keyboard shortcut (I did Shift+R): this will enable you to remove all tags for the selected item(s) by pressing Shift+R

5. Go back to the main zotero window and select 'My Library' (Brown folder). Click Command+A to select all items, then Shift+R (or right click < Zutilo < Remove all tags) to remove all tags in your library

6. Reinstate the tags you saved by going back to that new collection you made in step 1 ('saving coloured tags') and add back the tags to each reference in those subfolders. You can quickly add the same tag back to a list of items by draging a set of items to a tag in the tag selector on the left (as per https://forums.zotero.org/discussion/64065/tagging-multiple-items-at-once)

This seems to have worked for me at removing the hundreds of useless tags imported from EndNote and has synced across my desktop and laptop fine. Hope that helps!