Mike has nailed the California situation.
In more general terms, the requirements for new vehicles and vehicles operated on the road are often quite separate. In Canada, new vehicles must meet Canadian Motor Vehicle Safety Standards to be legal for sale
- a federal regulation. What actually operates on the roads, on the other hand, is governed by provincial regulations. The provincial regs (like state regs) are much less specific, and may not require equipment which is required by the federal laws on new vehicles.
It is common for newly required equipment to be specifically not
required on vehicles built before the date that the new vehicle requirement came into effect. The California breakaway switch is an example; for motor vehicles, features such as high-level brake lights
(CHMSTL) and - in Canada - daytime running lights fall
into this category.
I'm not aware of any requirement applying to trailers here in Alberta which is missing from my 1979 Boler's original equipment. But it had a breakaway switch stock, even then.