In one word, fundamentally. In many words, no one (and I mean literally no one -- past, present, or future) was, is or will be able to put it into words without ambiguity or total vagueness. That is simply the inherent limitation of language and rational "circuits". One has to "see" it for oneself.

Also, deep sleep cannot be experienced. Or better said there's nothing to experience. You have no mirror there, so to speak. The same is true for any method you choose in order to stop thinking. Be it meditation (hard AF) or entheogens (hit or miss). The best human is capable of is to catch the transition when mind kicks in and the "show" resumes.

That being said, you can "find" the "I" I'm talking about simply by observing your mind. You are that witness of your mind. But here you must be careful, because trying to consciously observe consciousness creates its ever subtler mind representations. Once you go "deep" enough and are able to catch the representations as they are created within mind, eventually it dawns upon you. And when it does, you won't neither miss nor mistake it.