It is possible to set or update ReadSecurity or WriteSecurity using REST API and JSOM both.
You need to make an
UPDATE request to above end-point with above request body. I have tested it using My SP REST Client.
More about ReadSecurity & WriteSecurity
WriteSecurity do not appear in response when you make a
GET request to the list end-point but it returns all properties those are mentioned in MSDN. I mean following end-point
But if use
$select operator in your query, then you will get the value of
WriteSecurity. I mean following
The reason is: these (
WriteSecurity) properties are not returned with the resource. See more on Getting properties that aren't returned with the resource.
Set value using
Get value using