Literally the index. If you track the NASDAQ as part of your index you must obey it's inclusion rules.

Funds often have institutional investors. Many of them Pension Funds (i.e. ordinary every day people) and when the institutional investors signed up, they didn't do so expecting Nasdaq rule change.

that is the whole point of an index fund - they buy whatever is in the index so you can get exposure to the total market. The scandalous thing is that an IPO'd company is going to have a lot of volatility for weeks to months after it goes public, so they typically do not allow any newly listed company to be included in the index for up to one year. This is for the benefit of retail. People have put their entire life savings into these funds because they are viewed as the optimal tradeoff between risk and return. Those people are now contractually obligated to either sell everything or expose themselves to spacex's IPO price movements.