We already have many pages that covers the same topic for D7 and D8 – and maybe there will be more versions supported in the future. Many of the topic pages even have the same URL pattern:
If you want to guide a user to the same documentation for a different version of Drupal, your best bet is “Related content” at the bottom. It doesn’t look good and it is a manual task adding and maintaining those links (example 1, example 2, …)
I guess this could wait for the “Documentation initiative” to start rolling, but I think it’s a pretty lowing hanging fruit that would benefit our users greatly – similar to.
Suggested (temporary) solution
If there exist version specific “duplicate” URL aliases (as the system-requirements example above), then we add a version switcher some where visible on the page – maybe in a fashion similar to symfony (example).
This requires that we are slightly better at setting good URL aliases and probably turning of automatic URL aliases completely – which is a good thing anyway.