Mac App Store Stops Working on Older macOS Versions

Mac users running macOS Mojave (10.14) and earlier versions are reporting widespread issues accessing the Mac App Store, with many unable to sign in or download previously purchased apps.
Based on reports compiled by Michael Tsai, the problems appear to have emerged around January 24, 2025, coinciding with Apple making changes to Mac App Store receipts. Users attempting to access the store on affected systems are receiving various error messages, including "An unexpected error occurred while sig