Where is the local.tex file described in lshort.pdf present on a Debian system?

Note it says "should". LaTeX predates the web and other forms of instantly accessible help like this site. The original LaTeX Book had the wording that was copied into lshort that the local installer "should" make a local guide detailing to end users at that site how latex had been set up and configured.

In practice almost no one ever did this even when tex installation was something that only computer specialists would do on some university system.

In fact one of the driving forces behind the lshort document was the fact that many TeX users had no local information at all (hence the slightly ironic entry for a "local guide" in the references).

In the good(bad) old days there were many different Operating Systems with their own idiosynchranites(SP) but now there are many fewer. My view is that the local.tex document was meant to be configured the peculuarities of these systems by the local sysadim (who had probably never heard of (La)TeX). In my time I have had to use at least six different OS's and have never come across a local.tex.

I think that the current equivalent of a local.tex is lshort.tex (texdoc lshort)