I think there might once have been two forms of the future, because they were mirror images in conjugating, and had slightly different meanings.
I/we shall, thou wilt/you will, and it/they will were “plain future,” it was just what would happen.
The mirror image: I/we will, thou shall/you shall, it/they shall, implied something of an obligation or statement of willing: I shall go to the store tomorrow because I always go on Thursdays vs I *will *go to the mall, and you can’t stop me!
These distinctions have pretty much died out these days, possibly because the contractions are all identical ('ll), but are still retained when we ask a question: what shall we do today? or legal papers when something is mandated: Records shall be kept on-site for a minimu of 3 years.
So: Thou shalt not … is an imperative rather than an expression of future tense.