Annex E (informative) Conformance with UAX #31 [uaxid]

E.3 R2 Immutable identifiers [uaxid.immutable]

An implementation may choose to guarantee that the set of identifiers will never change by fixing the set of code points allowed in identifiers forever.
C++ does not choose to make this guarantee.
As scripts are added to Unicode, additional characters in those scripts may become available for use in identifiers.