Edit: Changed the title from “Deprecate …” to “Remove …”.
Currently the OpenMP version in flang defaults to 3.1, and it reflects the degree of support of OpenMP, namely some later features are not implemented yet.
The lowest version that may affect the behavior of the compiler is 4.5. More specifically, the compiler will do the exact same thing for any OpenMP version equal to or less than 4.5, with the exception of the value of the _OPENMP macro. Any actual semantic differences are handled starting from version 4.5.
Since it is unlikely that flang will ever support any features differentiating versions prior to 4.5 (outside of the macro, such as diagnosing directives/clauses that were introduced in later versions of the spec, etc), I propose to simply remove any support for versions 3.0 and older.