CRM SDK Query where optionset values are valid

I am trying to create a query using CRMSDK where i'm trying to return a set of records where the field value exist in the list OptionSetvalues. The problem is that when u remove a value from the picklist, records that reference that optionset does not get updated and records get returned with the deleted optionset value.

ex

var query = from u in Context.SystemUserSet

                    where u.new_field exists in (optionset) ///something like this.