Have a look at the Drupal Upgrade page, which includes this:
UI and drush command for core upgrades from D6 or D7 to D8 - the plan
is for this to live in contrib for now and ultimately bring the UI
into core (a decision will be made shortly before RC whether it will
make it into 8.0.0) and the drush command into Drush itself
So since it is still in active development, it has not yet been added to core. In other words: consider this as a "didn't make it to D8 CORE yet".
Refer to the issue about "Plan for Drupal Upgrade 8.x-1.0 release" for what, IMHO, is probably the best place to see how this is going to evolve in the (near?) future ... Like will it make it into Drupal 8.1 "core"?
Note: The module's Project page title is "Drupal Upgrade", while de module's project page URL ends with "migrate_upgrade" (and not with something like "drupal_upgrade").