the online calculator I linked above publishes 1SImpson Index value. So, it has the more intuitive value, and calls it the "dominance index."
@vaughnbros Gini coefficient doesn't capture both elements tho  it only captures inequality, not "richness." Ryan was suggesting Gini Impurity, which is not the same thing. It's a totally different formula.
I also manually automated an excel spreadsheet to calculate the Simpson Index value. So I went through and calculated the Simpson Index value for:

every single month where there has been 3 or more Vintage Challenges (so, June, 2017through October, 2018)

for Quarters for the Power Nine Challenges (Q4, 2015, Q1, 2016, Q2, 2016, Q3, 2016, Q4, 2016, and Q1+April, 2017).

And I went back into some old metagame reports, and calculated the Simpson Index scores for those, including Q1, 2010, and MarchApril, 2009.
For all of those, the Simpson Diversity Index values match intuition from looking at the scores.
I will try to figure out the best way to share all of that information, but give away the most revealing bit of information, for all of those calculations, which is 25 values, the highest Simpson Scores (all above .30) were the two months before Mentor and Thorn were restricted. And the lowest score among those 25 calculations was October, 2018, at just about .10. This year had most of the lowest scores.
What is shows is that, from an objective metric, the metagame right now is the most diverse & balanced it has ever been, using data from the Vintage or Power Nine challenges.