It is unused (I couldn't find a bug for this).

// product_rw flags are writeable internal product flags.
//    They are like "manageable" flags but for internal/private use.
//    The list of product_rw flags are internal/private flags which
//    may be changed/removed in a future release.  It can be set
//    through the management interface to get/set value
//    when the name of flag is supplied.
//
//    A flag can be made as "product_rw" only if
//    - the VM implementation supports dynamic setting of the flag.
//      This implies that the VM must *always* query the flag variable
//      and not reuse state related to the flag state at any given time.
//
// Note that when there is a need to support develop flags to be writeable,
// it can be done in the same way as product_rw.

Further from [~stefank]
- The specification of product_rw is at odds with our current definition of product. product_rw are flags that are "writeable" by the managment API, but not listed by the getDiagnosticOptions() function. And, importantly, they are not supposed to be supported and can be removed any time. If we can create diagnostic, experimental, develop manageable flags this is not needed or wanted.