I'm one of the seeming few who paid to register it back in the Win9x days. Turned out that the key also works for the non-free Linux version (think I only ever bothered to set that up once). These days gzipped tarballs or squashfs with zstd are my go-tos. The former for speed with source trees or such, and the latter for filesystem backups or larger directories, because loop mounting it is so fast and convenient.