Prohibited was the wrong word, you're right. Institutional investors that run ETFs have become averse to dual class stocks ($SNAP, $LYFT) because they don't have the same voting power per share that they do with other share class setups. Indexes like S&P 500 have also stopped including listings that are dual-class -- this is changing though, see the articles I've linked below.
Dual-class share setups are typically prohibited by most ETFs. That's why companies that do want to have more than one share class will trade them under different tickers ($GOOG vs $GOOGL).
Edit: Prohibited was the wrong word, looked down upon was probably better.