Drupal 9 - Compatibility

Regarding #4
I don't know what is required to be able to get the node from the context in the class that needs to be modified.
Maybe use the trait like blockbase does.
https://api.drupal.org/api/drupal/core%21lib%21Drupal%21Core%21Plugin%21...

Regarding the patch I think it might still fail as the code is initializing $node to NULL but later is used as $node->isTranslatable()
when it still might be NULL.