Sebastian Rahtz writes:

: *LaTeX* is as described in Lamport's book, of which only the 2nd edition
: can be bought now. That describes LaTeX2e, ie standard LaTeX. The older
: LaTeX209 is deprecated, unsupported, etc etc.

I agree that LaTeX is defined by Lamport's book, 2nd edition.

The 2nd edition does call for the use of \documentclass.

Lamport does recommend the LaTeX Companion.  On the other hand, I
would say that standard LaTeX, hence, what I would today call portable
LaTeX is delimited by the commands described in Lamport's book.

Today certainly one cannot expect portability with all of the commands
described in the Companion.

I see a need for (1) a core and (2) a subset of the core defined as
portable.  (If the subset should turn out to be the whole, then that
is fine so long as it is understood.  But my expectation is that over
the years (2) should grow much more slowly than (1).)

                                   -- Bill