Hi Richard,
yes it is safe to delete them from the DU000020 table, but you should also check other related tables that hold product ID's version (DB_Upgrade, DU000030, and maybe more).
If you have GPPT installed, you can use the "Fix Utilities" in the database validation to check for old product entries and get rid of them.
Derek,
I've discovered an annoying bug in the new deployment package feature from GP 18.2.. when you set the new folder name where GP should be installed, it's not respecting it.. If I set the folder name to be 'GP', after the Workstation install, it ends up being GP2018.. I know this sounds like a minor issue, but it means also that if you have post-deployment scripts that are based on that path, they will fail.
If you run the GP Setup manually though, the default path folder name is defaulted to "GP"..