Dawn! HELLO!!!
Yes, the tool is still relevant ... more than ever ... I teach it all the time. It's called the Flesch-Kincaid calculator (after its co-founders).
Here's the URL: https://goodcalculators.com/flesch-kincaid-calculator/
As I probably harped on back then, grade level is NOT about getting 6th-graders to make donations.
Grade level is exclusively about how fast and easily I (an adult who might make a gift) can comprehend what you're trying to tell me: a 6th-grade level is faster than an 8th-grade level is WAY faster than a 10th-grade level ... and if you're trying to shove 12th-grade writing down my throat, you better be buying me a hot-fudge sundae with 3 cherries on top. And a day in rehab.
12th-grade writing ceases to be of any use after the 12th grade (unless you want to be a peer-reviewed scientist or college professor). For the general public, it's an insult to our attention spans.
Flesch-Kincaid standards ripple through the User's Manuals on our sub fleet, where sailors with high-school diplomas tend nuclear reactors and drive the boats. The people at the wheel are 18 and 19 years old (under senior supervision, of course). Thank you, Flesch-Kincaid.
Without Flesch-Kincaid, what a mess we'd be in.