Unlike Facebook, Tumblr doesn’t let you temporarily deactivate your account. Once you click the delete button, it’s gone forever. Another major thing to be aware of is that if you delete your primary blog, it’ll automatically remove all of your secondary blogs as well.
However, if there’s one thing Tumblr is known for, it’s screenshots. There’s no way to tell if other users have kept their own copies of your posts, but if anything you did on the platform ever went viral, it’s almost certainly available somewhere.