Handbrake offers the ability to convert copy-protected DVD's (aka "rip") too. It is not able to do this out of the box, but instead, relies on a specific library called libdvdcss. This library is installed by default when you also use VLC media player, but a version compatible with Handbrake is also available for download.
For Mac OSX, an installer was made available to easily install the library and have Handbrake use it, but due to the write-protection of system folders, introduced with macOS High Sierra, this method no longer works. As a result, it became difficult to use Handbrake. Fortunately, there are a number of workarounds and other methods of installing the library, which enables this functionality again.
Most methods found on the web nowadays involve in installing the library through Homebrew, which in itself is a proper way of installing it but involves downloading and installing additional packages (i.e. Homebrew and its dependencies), and for some (like me), this is not really wanted.
Luckily, the method of manually downloading the library (also known as "the old method") still seems to work and is valid even on macOS Mojave but with one difference: the location of the library on disk. Rather than placing the library in /usr/lib (which no longer works because of the write-protection of system folders), it needs to be placed in the folder /usr/local/lib. By placing the library there, Handbrake will detect it and the copying of copy-protected DVD's will work once more.
To get the library, open a Terminal and run the following commands:
cd /usr/local/lib sudo curl -O https://download.videolan.org/libdvdcss/1.2.11/macosx/libdvdcss.2.dylib
If the first command gives an error that it does not exist, run the following command to create it:
sudo mkdir -p /usr/local/lib
Note that the first time you run a command starting with sudo, you will have to enter a password of an account with administrative privileges. Subsequent runs of those commands will not prompt you for passwords (unless you close the Terminal window).
After installing the library, be sure to completely restart Handbrake (use ⌘ + Q when it's the active application). You'll know right away when the library works if you are able to see the previews when attempting to select a copy-protected DVD as a source.