Perhaps something like having forums set up similar to: (looking to the future
Versions 3.0 and older (EOL)
(separate forum with a set of sub-forum groupings:
Version 2.0s and older
As versions hit their EOL, their topics could be moved into the appropriate forum section. (And/or whole sections moved from the main support section to this other section for older versions...)
It would just mean a bit of housekeeping & moving the furniture around here every once in a while....