Earlier quoted context omitted.
Unicode's mission is to make every document "roundtrip-able". Even if a character is only used once, it should be possible to save a plaintext version of the containing document without losing any information. Roughly, I should be able to put a transcription of that one translation from the 1400s on Wikisource without using images. You may disagree with me, and that's fine, but it doesn't change Unicode's mission. Be…
If that was once its mission, it was clearly abandoned long ago. They rejected Klingon characters on the grounds that it has low usage for communication, and that many of the people who do communicate in Klingon use a latinized form. ꙮ seems to just be a fancy way of writing О. I haven't seen anything that says it has a different meaning. The arguments for excluding Klingon seem to apply even more so to ꙮ.
A weird solitary character from the 1400s isn’t subject to that, and even if it’s a mistake it’s probably not worth breaking compatibility at this point (I think the last such break with code points genuinely changing meanings was to repair a mistaken CJK unification some time in the 00s, and the Consortium may even have tied its own hands in that regard with the ever-more-strict stability policies).
Similarly, for example, old ISO keyboard symbols (the ⌫ for erase backwards, but also a ton of virtually unused ones) were thrown in indiscriminately at the beginning of the project when attempting to cover every existing encoding, but when the ISO decided to extend the repertoire they were told to kindly provide examples of running-text (not iconic) usage in a non-member-body-controlled publication. (Crickets. The ISO keyboard input model itself only vaguely corresponds to how input methods for QWERTY-adjacent keyboards work in existing systems—as an attempt at rationalization, it seems to mostly be a failed one.)