When I use that link (https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12) in Safari my computer opens the software update window of the control panel at attempts to locate the update before giving this error:
Update not found. The requested version of macOS is not available
The link I get from the "How to Get Old Versions of macOS" Apple Support webpage is slightly different:
https://apps.apple.com/app/macos-high-sierra/id1246284741?mt=12
... and that link just leads me to a dead-end page that sticks forever saying:
Connecting to the Mac App Store
..
To use the Mac App Store, you need Mac OS 10.6.6 or greater. For more information, please click here
I am using a MacBook Air running macOS11.1... so that isn't the problem, and there is no High Sierra update available on the App Store anyway (at least not that I can find).
Thanks