While it almost sounds good to prohibit taxing retirement savings and 401k, in practice it's written as a loophole that enables billionaires to accumulate immense untaxable wealth.
Prop 40 is a one-time tax, including a 5% tax on the amount exceeding $10 million in a Roth IRA. So a $12 million Roth IRA would have $2 million taxed at 5%, which is $100k — and again, that's a one-time tax.