Thank you to contributor @mibe for changing the IP address to an empty string instead of a dash (#2), entirely removing YOURLS' country code detection which may increase efficiency (#1), and anonymising the HTTP referrer as YOURLS may use that in future for its analytics (#3).
Installing
Install this release by decompressing the archive in your YOURLS plugins directory, and then enable the plugin in your YOURLS admin interface.
Upgrading
Decompress this release's archive, and replace the yourls-anonymise directory in your YOURLS plugin folder with the one you just decompressed.