My guess is that this was invented independently several times. I think 
I used it myself in the 70's (and not on UNIX), as soon as I had a text 
editor, because "XXX" was easy to search for and was not going to 
overlap with variable names, etc.

There's a discussion here: 
https://www.snellman.net/blog/archive/2017-04-17-xxx-fixme

Dan H.

On 9/3/20 4:35 PM, Warner Losh wrote:
> I'll also add that this seemed foreign when I had patches that had XXX 
> in them I submitted to the linux folks in the early 90s. It was second 
> nature in the BSD side of things. But I don't know if that's a 
> Berkeley thing or a Bell Labs thing Berkeley picked up...
>
> Warner
>
> On Thu, Sep 3, 2020 at 12:11 PM Warner Losh <imp@bsdimp.com 
> <mailto:imp@bsdimp.com>> wrote:
>
>     The earliest my quick grep could find was 4.0BSD. I didn't find it
>     in this sense in pwb, but it was a quick grep...
>
>     xxx is used extensively in prior versions, but there it's meaning
>     is 'placeholder' or 'don't care'. Mostly for /tmp/XXXX files, but
>     also for things like Jxxx handles all the jump commands or dates
>     of the form 24 Feb XXXX or stuff like that.
>
>     Warner
>
>     On Thu, Sep 3, 2020 at 11:34 AM Dave Horsfall <dave@horsfall.org
>     <mailto:dave@horsfall.org>> wrote:
>
>         For yonks I've been seeing "XXX" as a flag to mean "needs more
>         work" or
>         "look at this carefully, just in case" etc, and I use it myself.
>
>         Whence did it come about?  I think I saw it as early as PWB,
>         but can't be
>         sure.
>
>         -- Dave, wondering how many nanny-filters he triggered with "XXX"
>