Comments:

* For versions <=4, compatibility tests are exact.  Thus, a site running
  '4.6.20' would only see extensions that were tagged `<ver>4.6</ver>`
  but not `<ver>4.5</ver>`. This preserves existing behavior.
* For version 5, forward-compatibility is assumed.  Thus, a site running
  '5.1.0' would see extensions that were tagged as '<ver>5.0</ver>' or
  '<ver>5.1</ver>'.  Also, because `5.{$y}` is really a continuation of
  `4.7.{$z}`, we include extensions from "4.7".
* This assumes that we prefill the list of taxonomy terms (e.g.  "CiviCRM
  5.0" through "CiviCRM 5.36").

See also: https://lab.civicrm.org/development-team/Release-Management/issues/1