Homebrew 4.0, released in February 2023, changed the default so that homebrew/core and homebrew/cask are no longer cloned as local git repositories. Metadata comes from a JSON API instead, which cut the initial install from hundreds of megabytes and made 'brew update' dramatically faster.
The consequences show up in tooling. There is no local formula file to read, so scripts that pointed at Formula/foo.rb inside the tap path break; use 'brew info --json=v2 foo' instead. 'brew edit foo' on a core formula no longer opens a checked-out file unless the tap is present. Searching by grepping the tap directory finds nothing.
If you need the old behaviour, for example to develop a patch against a core formula, run 'brew tap --force homebrew/core' to clone it and set HOMEBREW_NO_INSTALL_FROM_API=1 so that Homebrew reads from the checkout. Expect the clone to be large and 'brew update' to slow down again. Existing installations that were upgraded keep their old taps until they are untapped.