Indices have been handling e.g. Berkshire Hathaway's two classes of listed stock for decades.
Some buy proportionately (i.e. if Berkshire Hathaway is 10% of a market and 15% of its value is in Class B and 85% in Class A--making up numbers for illustrative purposes--then the index puts 1.5% into B and 8.5% into A) and others defer to a single class based on judgement and rules (e.g. 10% into A). But it's a trivial problem of index construction.
I tried to look more into the decision. These seem to be the two relevant quotes:
>Companies with multiple share class structures tend to have corporate governance structures that treat different shareholder classes unequally with respect to voting rights and other governance issues
>S&P Dow Jones Indices also said that the S&P Composite 1500 indexes, like the S&P 500, follow “more restrictive eligibility rules,” such as positive earnings based on accepted accounting rules whereas its other index groups were “intended to represent the investment universe.”
So I suppose they aren't really "the very least picky possible investors". Their mandate seems to include actively curating the S&P 500, despite its tendency to be used by "passive" investors.
Some buy proportionately (i.e. if Berkshire Hathaway is 10% of a market and 15% of its value is in Class B and 85% in Class A--making up numbers for illustrative purposes--then the index puts 1.5% into B and 8.5% into A) and others defer to a single class based on judgement and rules (e.g. 10% into A). But it's a trivial problem of index construction.