Why does System V traditionally symlink /bin to /usr/bin

The short answers are, yes, it was done for compatibility (lots of programs referenced /bin/sh and /bin/ed), and in the early days /bin and /usr/bin contained totally disjoint sets of files. /bin was on the root filesystem, a small disk that the computer's boot firmware had to be able to access, and held the more critical and often-used files. /usr/bin was on /usr, typically an entirely separate, larger disk. /usr, at first, also contained users' home directories. As /usr grew, we would periodically replace its drive with something larger. The system could run with no /usr mounted, even if wasn't all that useful.

/usr's disk (or disk partition) was mounted after the Unix kernel had been booted and the system was partway through the user-mode boot process (/etc/rc), so programs like sh and mount and fsck had to be in the root filesystem, generally in /bin and /etc. Sun had even rearranged / and /usr so that a shared copy of /usr could be mounted read-only across a network. /usr/tmp became a symlink to /var/tmp. /var was either on the root filesystem or, preferably, on another partition.

I believe it was Sun that decided, at one point, that it wasn't worth heroically trying to have a system be able to come up if its /usr was trashed. Most users either had / and /usr on the same physical disk - so if it died, both filesystems were toast - or had /usr mounted read-only from a server. So some critical programs used for system boot and maintenance were compiled statically and put in /sbin, but most of the programs in /bin were moved to /usr/bin and /bin became a symlink to /usr/bin.

System V prior to R4 didn't even have symlinks. Sun and AT&T worked to combine SunOS and SVR3, and that became SVR4 (and Solaris 2). It had /bin as a symlink to /usr/bin.

So when that web site says "On SysV Unix /bin traditionally has been a symlink to /usr/bin", they really should have said "On System V Release 4 and followons, ...".