In my experience (non technical, my delelting post experience) when you create a post it propagates immediatly to any instance that is properly federate to your instance. This usually means same software and people from there following content from your instance. Then, with a lot of conditions to be fullfield, it propaged to partially federated instances. For example instance of another software that makes full federation more difficult, or instance that does a search on your specific post...

When you delete your post, it deletes automatically to any properly federated instance but that could be with a delay (a few minutes, or an hour) but the delete order has issues reaching all the other instances (the ones that were federeate when you create the post but not anymore, the ones from another fediware, ...)

You also have no guaranty that someone doesn't have your post in a backup they may use to restore a fallen instance, or leak through another way. As with every content on the internet, better consider anything once posted as know publically forever.