I realize that this brew tap has everything I need, and I don't need to install the old PHP extension using pecl or compiling from source.

https://github.com/shivammathur/homebrew-extensions

brew tap shivammathur/extensions
brew install shivammathur/extensions/[email protected]

Since I already have the old 7.4 mcrypt extension,

brew unlink [email protected]
brew link [email protected] 

Checking

php71 -m | grep mcrypt
mcrypt