How to store user meta data? I've seen that it was possible to store custom meta data in Drupal 7 but I didn't find the equivalent for Drupal 8 ? 

https://martsie.github.io/2015/03/04/drupal-how-to-use-user-data/

The data that I need to store is temporary user data that might differ from each users and it's not needed to put it as field (no sql query needed).
You can use the user.data service

\Drupal::service('user.data')-&gt;set('mymodule', $uid, 'some_key', 'some_value');


You can then retrieve it with the same service:

$some_key = \Drupal::service('user.data')-&gt;get('mymodule', $uid, 'some_key');