A pending Chatter post is a post that has been scheduled to be published at a future date and time. However, if the user who created the pending Chatter post is deactivated before the scheduled date and time, then the pending Chatter post will be canceled and will not be published. This is because deactivated users cannot create or edit posts in Chatter. References:
https://help.salesforce.com/s/articleView?id=sf.collab_scheduled_posts.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.collab_deactivated_users.htm&type=5