Billionaires including Peter Thiel, the tech venture capitalist, and Larry Page, a co-founder of Google, are considering cutting or reducing their ties to California by the end of the year because of a proposed ballot measure that could tax the state’s wealthiest residents, according to five people familiar with their thinking. Mr. Thiel, 58, who owns a home in the Hollywood Hills and operates a personal investment firm from Los Angeles, has explored opening an office for that firm, Thiel Capital, in another state, and spending more time outside of California, three of the people said. Other billionaires who appear...