I've not (yet) read 40+ messages in this thread.
I'd borrow a measure from vehicle licensing in what defines a "classic"
car.  Specifically any care 25 years old or older.
It's not a perfect line, but it is a relatively easy line.  I find that
a quarter of a century to be easier to remember than something else; 20
/ 30 years.
Of course, this is a guideline and things will fall either side thereof.
Just my 2¢ worth.
On 12/20/22 4:36 PM, Zane Healy via cctalk wrote:
  The original rule, back around ’96/97 was anything
older than 10
 years was on-topic.  I think the idea behind that is still valid,
 but these days a 10 year old system isn’t that much different from
 a current one.  I think at one point someone suggested it should be
 shifted to 20+ years. 
--
Grant. . . .
unix || die