It really is pretty crazy that some of the more esoteric forms of citizenship have never been rolled into "British Citizen". Almost all BOTCs were given the opportunity to become British Citizens, but not all, and they kept the original status around. BNOs are similarly a somewhat silly situation especially now that it's possible to move to the UK on the special BNO visa (which gives them different/better family reunion rights than normal British citizens). British Subjects essentially don't exist in practice, but they also haven't just rolled that into British Citizen status either (British Subject is the residual status of certain Irish born people who chose to retain the status - they have the right to live in the UK on the basis of their Irish nationality, not on their British nationality which is insane). There's just a perpetual allergy to just rationalising the whole setup.

It would be perfectly politically acceptable to just do away with the statuses that have fewer than say 5000 people and grant them all full-fat citizenship. Generally people who live in the UK are shocked when they find out that holding a British passport does not entitle you to the right to live in the UK.