Per that earlier documentation, Plantronics also uses the per-user ~/Library path, so you might also want to use the following search for your own local Library folder:
sudo find ~/Library -iname *plantronics* -print
That command differs from the earlier find command by one character; by the addition of the ~ tilde for the local path. In the Plantronics documentation linked earlier, the ~/Library looks in your local /Users/YourShortNameHere/Library folder. There can be other local ~/Library folders for other users, if they too were using the Plantronics software.
To find any files with that string anywhere in any of your local /Users's per-user files, including in their local /Library folders...
sudo find /Users -iname *plantronics* -print
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