Apple has started making the following macOS installers available outside of the Mac App Store (MAS).
For each listed OS installer, Apple has direct download links via their relevant KBase article for InstallOS.dmg or InstallMacOSX.dmg disk images.
In turn, these disk images contain installers named InstallOS.pkg or InstallMacOSX.pkg.
These installers do not themselves install the relevant version of macOS or OS X. Instead, they install the Install.app for that macOS or OS X version into the /Applications directory.
Once the relevant Install macOS or OS X app is available, it can be used to install that OS.
The installers for the following macOS versions are still available via the MAS.
They can also be downloaded on macOS Catalina using the softwareupdate tool.