Today, the California Supreme Court decided a case of interest to all of our friends who work for the government, City of San Jose v. Superior Court, S218066:

Here, we hold that when a city employee uses a personal account to communicate about the conduct of public business, the writings may be subject to disclosure under the California Public Records Act (CPRA or Act).  We overturn the contrary judgment of the Court of Appeal.

CPRA (Govt. Code §§ 6250 et seq.) is California's version of the Freedom of Information Act.  Many states have similar laws.

No big surprise here.  You can't evade the FOIA-type laws by using a private server.