Well, the directives of the SG16 Direction Paper says to avoid excessive inventiveness....

Because the unicode character database properties have pervasive uses in Unicode (staring with Normalization, and branching out from there), it would be incredibly hard to properly provide all of the C++ algorithms a chance to override the database used. You'd have to make sure every single algorithm can receive the database; or, define a "weak symbol" from the implementation which you then plug with your information. I don't know if the Timezone Database (tzdb) in chrono/date can help shed some light on how this might be done in a Modern C++ API without breaking implementer backs; looking there might be a good start.

I'm still unsure if it's necessary but if it's going to be customizable we'd better make sure it's done right...

Best Wishes,
JeanHeyd