Bingo! “Mac OS Standard” is Apple’s name for the old HFS (not HFS+) file system.
Apple started the process of dropping HFS support in Mac OS X 10.6 (Snow Leopard) - volumes could only be mounted read-only. Support was officially dropped in macOS 10.12 (Sierra), but unofficially kept working until 10.14 (Mojave). Starting with 10.15 (Catalina), HFS volumes are not mountable at all.
If you need to access the content of an HFS volume on Catalina or later, there are two options I can think of:
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Install macOS 10.14 or older in a virtual machine or an emulator. Mount the ISO file or optical disc from there.
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Use the open source hfsutils software package. It is part of most Linux distributions. It is available for macOS via Homebrew.
Note that hfsutils does not actually mount the volume like a normal file system, but does provide a command-line mechanism to walk through a volume and read/copy files. I find it very awkward to work with.
Sadly, neither of these options is very convenient if you just want to read a few files from a CD-ROM or a CD image.