ShittyDarkSouls

You need to have optional areas to reward exploration, but that's not the equivalent of "the whole game is optional". Even BotW where 99% of the game is completely optional you and everyone else still have to complete the following steps: activate the first tower, clear 4 shrines, talk to the king, kill Ganon. Minecraft for example is an "everything is optional" game and plenty of people would argue it's less of a game and more of a pure sandbox because you have to set your own goals and do your own thing and you're done when you don't feel like playing anymore. You can't "finish" Minecraft.

A game designer will always need to create some mandatory parts if they want the game to have an end state beyond "you died". Mandatory parts don't invalidate exploration except when "everything" is mandatory.