" Unless $HOMEBREW_NO_INSTALLED_DEPENDENTS_CHECK is set, brew upgrade or brew reinstall will be run for outdated dependents and dependents with broken linkage, respectively.
Unless $HOMEBREW_NO_INSTALL_CLEANUP is set, brew cleanup will then be
run for the upgraded formulae or, every 30 days, for all formulae."
However I don't think even these flags are sufficient. I think to really ensure nonbreaking behavior you have to use a brew pin command on each and every formula you have installed on the system.Really annoying defaults considering brew is a developer facing program not a consumer facing program where the tradeoff between security and functionality makes more sense.
This is why I still reach for conda whenever possible. I am at least prompted before 100 things are installed.