In the specification it is detailed that the reason is an optional string, and there are a set of pre-defined values, but I do not think it is clear that free-form values are intended to be supported as well. This has resulted in some SDK development which restricts the reason to an enumeration and prevents conveying reasons more specific to the vendor.

A current example of this in the python SDK: https://github.com/open-feature/python-sdk/blob/d1f27e3278964c2d81b780db97deace87d02cae1/openfeature/flag_evaluation.py#L21

(A different issue will be opened for that specific instance.)