Reinstall MacOS High Sierra on an 2010 iMac in 2025

Apple has a multi-stage device/OS support decommissioning and at the moment High Sierra is the oldest MacOS you get in the App Store.

However, Catch-22, to be able to install it you need a working Intel Mac which will be less and less likely as Apple progressing with M-series chips.

The workaround that solved it for me:

Download an .iso file that has the Install macOS High Sierra application (archive.org or torrents - yes, not an ideal source, but other ways did not work for me and I could not find an Apple image download that has the app).

cd /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/

# To be able to run the next command, otherwise zsh kills it:
codesign -s - -f createinstallmedia

# Make sure to replace /Volumes/usb with your USB thumb drive path:
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/usb

The workaround above I found on Reddit after few hours of experiments with other approaches and tools.

After having the USB bootable drive you can reinstall your old Mac as usual.