The idea is still around! Apple’s APFS file system (and HFS+in its later days) support sort-of transparent compression, and on all its platforms most system files - the ones that don’t change much - are compressed to save space for user files. There’s surprisingly little documentation about this.

There’s a third party tool you can use to compress files yourself: https://github.com/RJVB/afsctool

It looks like the technical details are in this pdf: https://developer.apple.com/support/downloads/Apple-File-System-Reference.pdf