tar.gz = tgz. the ball part is the compression algorithm. tar.bz2 tar.xz. still tarballs. still common. the slang has sorta faded away, but the principle is the same. unix law. layer processes in the architecture. tar is the filesystem. the ball is the packing.

I've got plenty of tarballs to go around lol https://www.vaughnnugent.com/resources/software/modules/VNLib.Core

Be aware tgz is often an extension used as a Slackware package as the output of a packageName.slackbuild script, as opposed to a .tar.gz file. If you untar a tgz file and there's no source, you may have a Slackware package, not a source tarball. That said, some of them are source packages anyway and have everything you need.