After upgrading from drush 7 to 8 with multisites, using drush from site directory cannot find drupal 7 (Windows)
It appears that drush 8 does not support drupal projects that were not built with composer. Drush 7 does not have this requirement. I added this composer.json to the drupal 7 root directory:

{
  "require": { 
    "drush/drush": "~8.0"
  }, 
  "extra": {
    "installer-paths": {
       ".": [ "type:drupal-core" ]
    } 
  } 
}


I found this in an issue for project drupal-finder, which seems to be used by the Drupal 9 Drupal Launcher. I'm guessing that drush 8 has similar code.