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Drush 8.1.11 incompatibilities with Drupal 8.4.x as of 02.05.2017 - Symfony components updated from 2.8 to 3.2.6 #2738
All Symfony components in Drupal 8.4.x have been updated from 2.8 to 3.2.6.
There are some API changes that have to be made to drush as well, because drush is extending from some Symfony classes.
An example where drush is not working anymore is site install:
Drush 9 supports only Drupal 8 according to https://github.com/drush-ops/drush/blob/master/docs/install.md