You can only install supported versions of PHP with brew. However, there is the tap shivammathur/php which can be used to install unsupported version of PHP.
brew tap shivammathur/php
brew install shivammathur/php/php@7.3
brew link php@7.3
The first step only needs to be done once. After adding the tap, you can install PHP version
You can also edit the formula and re-enable it. These steps worked for me:
brew edit php@7.4
# Look for disable! date: "2022-11-28", because: :versioned_formula. Change 2022 to 2023
brew install php@7.4