I wrote some unit test specifing the behaviour.
I think only Substask with a Status != Done and an assigned user should be taken into account. The search for "nobody" should not reveal tasks with assigned users when these tasks have a subtask without an user.
send direct messages. Need to make these database changes:
INSERT INTO settings VALUES ('integration_slack_webhook_channel', '');
ALTER TABLE project_integrations ADD COLUMN slack_webhook_channel text;