Discussion about why GNU/Linux system upgrades cause old programs to break

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You wrote on another list,

  > My specific experience was with old versions of Emacs on a typical
  > GNU/Linux system.  I keep past major versions on my development
  > system, for the purposes of investigating when a particular bug or
  > feature started happening, and it is common to have some of those old
  > versions stop working after a system upgrade.

Can you tell us the specifics about some of those incidents?  I'd like
us to see if by changing some practices (in Emacs or in GNU/Linux
distros or wherever) we could improve future compatibility.

"Old versions stop working" could mean that old executables no longer
run.  Or it could mean that old source trees do not compile.  To help
us understand how to prevent such problems, could you please make a
point to distinguish those two kinds of problem?

-- 
Dr Richard Stallman (https://stallman.org)
Chief GNUisance of the GNU Project (https://gnu.org)
Founder, Free Software Foundation (https://fsf.org)
Internet Hall-of-Famer (https://internethalloffame.org)