It may or may not be legal, but IF the measure qualifies for the ballot and IF it is passed, the tax would be imposed on bogey-billionaires who were CA residents on 1/1/26.
It's still theft, and it would still be one-time $$$$ funding ongoing programs. If it passes, I'm sure that in a year or three a new "one-time" tax will be proposed on 100-millionaires, 10-millionaires, or even 1-millionaires. Somewhere in one of the latter two brackets, ordinary home owners in the SF Bay Area and the Land of LA might wake up and realize, "Wait! That hits me!"