Rather, the convention was that /sbin was for static binaries so that the system could still be fixed online if the dynamic linker got hosed. It's not about /usr not being mounted, but /lib/ld-linux.so not functioning correctly. For that reason, glibc still ships (or used to ship) an sln binary (static ln), and Debian still offers sash (stand-alone shell): so you could at least try to restore the dynamic library link farm by hand.

But I have only ever seen historic references to that argument, from back when dynamic linking was scary and unreliable. I certainly have never encountered that situation in almost 25 years of using Linux.