You could try just deleting them using the computer OS- (mine is Windows-10)
My Copyright presets are located in - C:\Users\My Name\AppData\Roaming\Adobe\Lightroom\Metadata Presets
Find the location of your Presets folders from the Preferences.
Or- to do it within Lightroom:
1. In the "Preset" box of the Metadata panel, Choose "Edit Presets..."
2. In the Edit dialog, choose a preset you wish to delete from the drop-down menu.
3. Click the drop-down menu a second time this time selecting "Delete Preset named....whatever you selected in Step2" .
Thanks for getting back to me. I am on a Mac but was able to follow your suggestion. Unfortunately deleting the metadata preset did not help. I still get that drop down list when typing into the Copyright field. Seems like it is cached somewhere as opposed to showing on only the Copyrights in use which is what I'd expect.
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Go to Catalog Settings > Metadata and click Clear All Suggestion Lists. This will clear the auto-complete suggestions for all metadata fields, not just Copyright.
Thank you John (again).