A new initiative is currently being proposed in California:
On Aug. 9, 2021, venture capitalist Timothy Draper filed a proposal
with the California attorney general’s office for an initiated
constitutional amendment that would prohibit public-sector workers
from forming unions. If supporters gather enough valid signatures, it
will appear on the Nov. 8, 2022, ballot.
Are there countries/states that ban public sector employees from forming a union, but still allow private sector employees to form one? Or would California be the first such states if this initiative manages to pass?