I recently upgraded my Mac from 10.13 to 10.14 and I can't seem to find my Homebrew. /usr/local is practically empty; just MySQL remains. Does brew get removed after an OS upgrade? Should I install it from scratch? What about the packages I installed using homebrew on the older version?
You should run
brew update and see what happens. If Homebrew refuses to run, reinstall it using the command at https://brew.sh.
You should then run
brew upgrade to make sure you're running the latest packages. If
brew list shows no packages, then it looks like you'll have to re-install these packages.