Robinhood Markets Inc.’s shares fell Thursday, after the financial-services platform once again missed out on membership in the S&P 500 index. But investors should perhaps focus more on President Donald Trump’s tax-and-spending megabill, which could have much bigger implications for the company.

Broadly, the legislation makes President Trump’s 2017 tax cuts permanent, extending current income-tax rates. It also adds some new breaks while making cuts to federal healthcare coverage, food-aid and student loan programs.