How to define a coptic subset (or, should we change the Greek subset)?

@irenevl @moyogo @tiroj We are adding a coptic subset (referenced in https://github.com/google/fonts/pull/3324)

In early Unicode, Coptic characters (`3E2` thru `3EF`) were added to the Greek block (https://en.m.wikipedia.org/wiki/Greek_and_Coptic) and our Greek subset, and this was carried through ~11 years ago into the `greek.nam` GF API subset

https://github.com/googlefonts/gftools/blob/main/Lib/gftools/encodings/greek_unique-glyphs.nam#L115-L128

@marekjez86 proposes dropping those characters from that subset, and adding them to the new `coptic.nam` subset. However, it seems these characters were added initially because they are 'shared' or 'unified' with Greek in some way.

If a "greek" font includes them, should they be delivered as standard? Would they be kerned or otherwise be included in features that will be broken by sharding them into a 2nd subset?

Is it common to include them?