feat(openenum): add is_open boolean slot to enum expressions

linkml_model/model/schema/meta.yaml

@@ -650,6 +650,29 @@ slots:
     exact_mappings:
       - cdisc:PermissibleValue
 
+  is_open:
+    domain: enum_expression
+    range: boolean
+    description: >-
+      Whether the enumeration is open, meaning that values outside of the set of
+      permissible values are allowed. If true, the enumeration is open and data values
+      that are not in the set of permissible values are permitted (though they SHOULD be
+      drawn from the permissible values if available). If false (runtime default), the
+      enumeration is closed and data values MUST be drawn from the set of permissible values.
+    ifabsent: false
+    comments:
+      - An open enumeration is analogous to the `allow-other="yes"` construct in the OSCAL Metamodel.
+      - When an enum is open, the permissible values are advisory - they document the recommended
+        or expected values but do not constrain the range. In JSON Schema this maps to a plain string.
+    see_also:
+      - https://pages.nist.gov/metaschema/specification/syntax/constraints/#allow-other
+      - https://github.com/linkml/linkml/issues/127
+      - https://github.com/linkml/linkml/pull/3712
+    in_subset:
+      - SpecificationSubset
+      # BasicSubset
+    status: testing
+
   enum_uri:
     aliases:
       - public ID
@@ -2782,6 +2805,7 @@ classes:
       - code_set_version
       - pv_formula
       - permissible_values
+      - is_open
       - include
       - minus
       - inherits