Happy legacy booting, and may your old 2011 MacBook Air run forever.
Maybe you are trying to breathe life into a 2010 MacBook Pro that slows to a crawl under Catalina. Maybe you need a legacy version of Logic Pro or Adobe CS6. Or perhaps you are setting up a Hackintosh and need a vanilla installer.
The problem? Apple has removed Sierra from the "Purchased" section of the App Store. Finding a clean, unmodified (for virtualization or PC booting) or DMG (for USB creation) can feel like hunting for digital treasure.