This hasn't had a genuinely useful purpose for many years. On a modern package-managed Linux distribution, both / and /usr are under the control of the package

This hasn't had a genuinely useful purpose for many years. On a modern package-managed Linux distribution, both / and /usr are under the control of the package manager, and separating them doesn't make sense since they are modified in lockstep, and used as a coherent whole. And while in the Linux world we're busy "unifying" these locations, other systems can have the whole system on ZFS, where these locations can be in separate datasets if desired, and the whole lot can be snapshotted at will. Since it's all in a single pool, there's no need to even consider splitting it.