Readability Index Software for Mac

I used to have a “Readability Index” tool that worked with WordPerfect. Yes, that long ago. It was free and simple to use. Is there such a product that works with Mac OS and Scrivener? I don’t need a “grammar checker.”

The last time I needed to check the readability of texts I used the GNU package style and diction

It isn’t integrated with Scrivener but does include better readability metrics than those implemented by Microsoft in Word. There is a brew formulae for macOS for it.