Can't run Homebrew anymore
It looks like the Xcode12 install Command Line Tools which have newer ruby framework and your Brew version is not compatible with this one.
Looks like the issue started when I upgraded to Catalina. For some reason (or maybe it's normal) Xcode was not up to date. Found that when I was not even able to run python3. So I move ahead and installed Xcode 12 and homebrew later:
brew update
brew upgrade
It took a while but homebrew now is up and running:
$ brew --version
Homebrew 2.5.2
Homebrew/homebrew-core (git revision 0dad6; last commit 2020-09-27)