There is no need to include in a font any instruction about upper and lowercase for the usual A–Z/a–z characters.
But what about these uncommon glyphs for African languages, phonetic support and transliterations? Must I include some OT instruction to say that uni0195 is the lowercase for uni01F6? Or this is known by client application due to its Unicode support?
A side question: if the relationship between upper and lowercase is informed by Unicode, how to handle pairs of glyphs with no codepoint? Say, Gtilde and gtilde or Eacutedotbelow and eacutedotbelow. As they are out of Unicode specification, will the client program or OS understand their upper/lowercase relation?
This question does not refers to small caps/all caps/petite caps control. I know these are handled by
pcap features (or, to petite caps, a Stylistic Set as they are still not widely supported).