Wow, this is going to be a great read. First impressions: pretty decent comment to code ratio, only about 30k lines of actual code, all of the code uses carriage-return newlines (though who knows if that's an artifact of archival) as does FC.ASM for its find_eol subroutine, some files include what looks like manually-maintained revision history, and right there in STDSW.ASM we have feature flags.

One interesting comment in DIR.ASM:

    ;       DS:SI Points to asciz string of path (no leading '/').

and in SYSCALL.txt's documentation, everything uses forward-slashes as directory separators. EDLIN.ASM and FORMAT.ASM on the other hand make clear that the directory separator is backslash on PC;

    mov     ax,(char_oper shl 8)    ;get switch character
    int     21h
    cmp     dl,"/"
    jnz     slashok                 ;if not / , then not PC
    mov     [dirchar],"\"           ;in PC, dir separator = \
    mov     [userdir],"\"
    mov     [optchar],"/"           ;in PC, option char = 

Looks like early DOS supported both?