First, is a tweet even a thing you can own, legally? You can own the copyright and you can enter into a contract where you promise not to delete it, but a tweet is ultimately something that is displayed and processed by, and exists at the pleasure of Twitter; they're not a party to your NFT shenanigans and therefore not bound to any of it.

As for the hypothetical contract not to delete it yourself, you'd probably have a good argument that there was no clear meeting of the minds - without an explicit description of what the author intended to be bound to by selling a NFT you'll have a hard time getting the courts to enforce anything about it.