The issue with difficulty settings is that there is no 'standard': You're right on the mark in this way, with how it's unclear what difficulty you need to choose for the game to require you to use all the mechanics/systems.

Making a standard that sticks is impossible, AAA developers let alone indies wouldn't agree to much, and even if you could make a standard it would be a limit to the artform, putting it into a box of "This is what a game must be"

The ideal would be for developers to trust themselves to set a fair difficulty for the game without a setting. If a developer feels an unbearable need to have a setting anyway, a good compromise would be for the difficulty menu to VERY PRECISELY detail what it changes (but even this is flawed because you haven't played the game yet and likely won't understand the details).