The Crimson Binome
Hero
If a level 13 wizard has only used their level 7 spell slot, then generally speaking, they still have most of their mojo left and should still be just fine for the foreseeable future. But they can't cast another level 7 spell, no matter what, until after the next long rest. If the character acts on the less-specific knowledge that they still have most of their power, then they should act as though they can cast another level 7 spell, even though they can't. And what, then? Should they just act surprised, when they can't pull off another teleport, even though they could fire off half a dozen fireballs?First off let's talk a bit more about the idea that it's incredibly difficult to play the game if spell slots aren't character information. A character doesn't have to know exactly how many spell slots are left or even that spell slots exist in order to have a general awareness of how much magical power he currently has left. It's that general awareness that is required. The player can translate his exact knowledge of how many spell slots are left into less specific character knowledge in order to easily play the game.
The rules of the game reflect the reality of the game world, but they are limited in how much information can actually be conveyed through the printed medium of a game that needs to be playable without a computer. The actual reality of the game world is more complex than the rules of the game, not less.