/sbin, /usr/sbin is for binaries that need root. You put them in separate directories so their permissions all match up, and so they don't show up when completing in bash.
The paths without /usr - /bin and /sbin - are available from the get go. It is the very first partition that is mounted, and what is guaranteed to be available if you do "init 1" or boot in single user mode. You can also do fsck from there (assuming the boot partition is not damaged). I don't know how this integrated with initrd (initramfs wasn't a thing yet). I think there was only one "base system" - either initrd was very basic, or the whole base was in initrd, or something similar.
The paths with /usr were managed by the package manager. Word of mouth was: don't install anything manually there. If you do (via make install), keep around the source so you can do make uninstall. But better install to /usr/local or /opt.
I also got this explanation, but it never made much sense to me. First of all, the binaries there are executable by everyone anyway. Second, it really doesn't matter that they show up during completion. Third, many of them work fine and are quite useful without root! I don't recall the specific examples that bothered me (/sbin and /usr/sbin have been in my PATH forever now), but I think it was something like ifconfig or ping.
It's more complicated than that - many can do a subset of useful things without root.
Often they can read things as a normal user - things like `apt` or `sysctl` can show you information about your current system, but will only be able to change it as root.
And even something like "shutdown" might be usable for a locally logged in normal user on a systemd system - or it might not be, depending on local configuration.
Finding things that actually always "need root" for everything is kind of hard, even discounting "print help" as a useful thing in its own right. And if you only came up with "chcpu" and "switch_root"... would you really want to have a top-level directory just for those? Plus the historical location for some things is in /sbin, so moving them out has a compatibility cost.
Tbh I find the only winning move here is not to play. There are so few binaries that are actually only useful to root that they don't really hurt in tab completion, and they could always grow non-root accessible features.