After Homebrew remove the php@7.0 formula prematurely, I undertood that other developer, like me will need this feature to keep using 7.0 version. So, this is a simple tutorial with file to install the formula in you MAC .
Download file php@7.0.rb and move this to you Homebrew Formulas folder.
My default path is this one
/usr/local/Homebrew/Library/Taps/homebrew/homebrew-core/Formula/
After that you will able to run brew install php@7.0 again .
If you have some trouble to link with this version, you can use this command.
brew link php@7.0 --force --overwrite
Ty.