[opk]

The one time I came across upx, it was used on some malware. It was on a programme named gnome-pty-helper in a user's .config directory that was installed in cron and set to phone home to some locations that were stored in clear once upx had been used to unpack it.

Years back I used gzexe and also some pkzip based thing on DOS. On a modern system, you're better of enabling filesystem level compression which also won't break OS paging if the executable is run more than once.