The urls’ for updates are tied to the CORE_COMPATIBILITY string in drupal core (7.x / 8.x etc)
http://updates.drupal.org/release-history/views_slideshow/7.x is the url that returns the 7.x metadata for that project.
When we have modules that support *multiple* major versions (8.x/9.x etc) then we’re going to have to serve metadata for *both* versions so that we can cross major version boundaries.
7.x urls would remain unchanged:
8.x urls would remain, and would contain both 8.x and 9.x data.
The 8.x urls could stay forever, and contain everything from 8.x->9.x until such time as we rewrite the updates api.
Alternately we could change the url part from 8.x to something more universal like