...yes, I know in 100 minutes the latest installer will be macOS Big Sur (11.0).

#rollseyes

Not sure about using softwareupdate to download 10.15.7, but in case you need it, I recommend installinstallmacos.py:

GitHub - MacAdmins Scripts

softwareupdate --fetch-full-installer --full-installer-version 10.15.7

Just used above command to download 15.7.02

Downloading and installing 10.15.7 installer is what I get when i run the sudo softwareupdate --fetch-full-installer --full-installer-version 10.15.7 @donmontalvo

This is what I get when attempting to download the full installer today.

softwareupdate --fetch-full-installer --full-installer-version 10.15.7
Downloading and installing 10.15.7 installer
SUPreferenceManager: Failed to set object of class: __NSCFConstantString for key: LastRecommendedMajorOSBundleIdentifier with error: Error Domain=SUPreferenceManagerErrorDomain Code=1 "(null)"
Install failed with error: Update not found

It’s been hit or miss, I eventually had success using the App Store to trigger Software Update. It now seems to be offering the full installer. It failed several times and had to restart the download. Luckily it picked up where it left off. Was getting the same error with the terminal common on two different machines.

Used the bellow link to trip it.
https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12

Still can’t get either the terminal command or the above method to work on one machine.

I was able to get it from AppStore/Software Update on a computer that still had Mojave on it. Took three tries to get it without errors but it did finally work.

installinstallmacos.py has been much more reliable for me that the App Store or softwareupdate --fetch-full-installer. Just be sure to pull the most recent version if you have an older copy as Apple made some changes to the catalogs for the BigSur betas.

sudo softwareupdate --fetch-full-installer --full-installer-version 10.15.7 

still works, 8/31/21