can no longer run either brew upgrade or brew update: both abort with
Error: homebrew-core is a shallow clone. To `brew update` first run:
git -C "/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core" fetch --unshallow
MacBook Air (2020 or later)
Following assumes Monterey or earlier, and not Ventura…
What happens after invoking the specified git command?
Failing that, try brute-force updating Homebrew core:
cd /usr/local/Homebrew/Library/Taps/homebrew/
rm -rf homebrew-core
git clone https://github.com/Homebrew/homebrew-core.git
Failing that, try deleting the contents of /usr/local/Homebrew entirely, and re-downloading Homebrew, and testing again.
If you’ve gone through all that, check with the folks that support Homebrew. This is their app, and they know it best.
Can't update or upgrade Homebrew