truss was converted to use ptrace in 2007 with 6.4 being the last release to use procfs to do it.
I untarred 6.4 base, mounted devfs and procfs and chrooted inside. "truss /bin/echo" hangs in the kernel with "pseudofs" as wait channel.
I downloaded a 10.3 image and performed the same test on said kernel with the same result. In other words while I don't know when the feature got broken, it was already broken in a release which is no longer supported. As such I don't believe there are any users and the feature should be eliminated.
truss on patched kernel:
$ truss /bin/echo
truss: PIOCBIS: Operation not supported
truss: cannot open1 /proc/622/mem: No such file or directory