I wrote a little script to clean up the files of autounmask. My package.keywords folder contained a lot of these. The script puts all masked packages together in one file and deletes all, that are not installed anymore. Perhaps you can ship it with autounmask. Reproducible: Always Actual Results: The script works fine for me, nobody else tested it yet.
Created attachment 215257 [details] the script
Created attachment 215261 [details] Version 0.02, supports also the /etc/portage/package.unmask file
package no longer in tree