Expired Certificates Breaking macOS Installers →

Apple Support:

In certain circumstances, such as when an installer is damaged, you’ll see an alert that says the application can’t be verified and may have been corrupted or tampered with during downloading. Or it says that the package was signed with a certificate that has expired and may not be authentic.

You should download the installer again. If the new installer is also affected, please contact the maker of the software for help.

If this happened with a macOS installer, you can find the latest download links in the installation guidelines for these macOS versions.

Apple has the last six releases up for download:

Earlier versions are not available for download, which really bothers me, but such is life with macOS.