Should be possible: https://support.apple.com/en-us/HT208891 https://support.apple.com/en-us/HT208891 I've done this to install a beta, but not to try to inst

Should be possible: https://support.apple.com/en-us/HT208891 https://support.apple.com/en-us/HT208891 I've done this to install a beta, but not to try to install an older macOS version from a newer release. If for some reason, the Mojave installer won't launch from Catalina, then you could create a bootable Mojave installer: https://support.apple.com/en-us/HT201372 https://support.apple.com/en-us/HT201372