Does Big Sur brick a MacBook Air 2014?
https://support.apple.com/en-us/HT211238
Supported MacBook 2014 Airs from the Apple knowledge base article:

MacBook Air (13-inch, Early 2014)


MacBook Air (11-inch, Early 2014)

If the install caused issues with listed supported Macs, you'd be able to raise the issue with Apple.
Anything further would be speculation.