Catalina boots and runs on HFS+, but there have been reports of some incompatible apps. I’ve not had any trouble myself but I haven’t used it as my daily driverJust so you know, this will not work on Catalina. The new Volume Group concept, which splits the boot drive container into separate System and Data volumes (with the System volume being read-only) absolutely, positively will not work on HFS+. The CCC developer has made this clear in his blog. You can still use HFS+ on drives that hold data, and TM still requires HFS+ until we hear differently.