Foxley gives the manual procedure for sizing the lost+found directory on the aforementioned page 52.
I have the 1986 edition of Fielder's and Hunter's UNIX System Administration and it does not mention any such command in its discussion of lost+found in chapter 3. It references AT&T Unix System 5 Release 2 (or 'UNIX 5.2' as the book puts it).
But Google Books tells me that their later 1991 update, referencing Release 4 and with an updated title to match, does indeed mention mklost+found. So that looks like something that appeared in Release 3 or 4.
I used to port Unix for a living (V6 thru SysVr3) I know it was in Vr2, and I have a man page that says "(last mod. 1/15/87)" which would likely make it SysVr1 or earlier
My guess is that it was undocumented until Release 3 or 4, so the book authors simply never found out about it. Andrew Tanenbaum knew about it at the same time as the first Hunter and Fielding book, but likely this was word-of-mouth stuff.
The ext-family filesystems provide the mklost+found command to tap into this call if you need to recreate the lost+found directory specifically.