Here is a proposed feature for the DMG:
Apple does not officially host old DMG installers on its public website. Legitimate sources for the 11.2.1.dmg historically included: macos big sur 1121 installer dmg
This is the official command-line tool provided by Apple. It creates a valid installer but typically requires running via Terminal (instructions included below) as Apple no longer provides a standalone .dmg for direct double-click installation. Here is a proposed feature for the DMG: