This is very interesting. However, it doesn't appear this rule is applied consistently as Facebook is in the S&P500 and is probably the most infamous example of a multi-class share structure.
> Companies with multiple share classes already in one of the indices will have until September 2022 to change their structures or face removal.
> For existing constituents, the rule will apply with effect from September 2022, thus affording a five year grandfathering period to allow constituent companies to change their capital structure if they so wish.
From my reading, S&P has not yet announced a similar policy.
[1] https://research.ftserussell.com/products/downloads/FTSE_Rus...