The fact that you can tell when I use "it is" or "its" incorrectly is confirmation of redundancy. If there was no context telling you which it should be, you wouldn't know; and if there is context telling you what it should be, there is no need to write the apostrophe because you already know what the writer intended.
That much is fairly obvious. Further, data compression can give another insight into this. Standard text compression yields a much higher compression ratio than 50%. If I compress your question with "gzip" (a standard compression utility), it goes down from 770 bytes to 440 bytes (57% of the original). This is fairly bad, but this is with zero pre-existing knowledge -- something humans have loads of when processing text.
If I prepend what is currently on Wikipedia's front page (some text about "Suillus salmonicolor", some fungus), compressing your question takes an additional 378 bytes (49% of the original), probably because it can replace words like "and" with a very short code without having to pre-define it.
It doesn't end there either: trying to remove redundancy (e.g. writing words in conjugated form is almost always unnecessary; as are words like "she" in the phrase "Sarah went to the store when she needed something" ("Sarah went to the store when needed something" gets the message across; or at least "he" or "it" (both are a character shorter) would suffice).
I have often wondered how much redundancy is in languages: both in its basic form (sentences as an abstract concept) as well as in its representations (written characters or spoken sounds). I've come to terms with it because it must be for error correction. I probably wouldn't catch half of what my aunt is telling at the Christmas dinner table if not for that.
A related experiment that ran in 2013 looked at how much of a sentence you can hide before it becomes unreadable: https://lucb1e.com/rp/js/read.html For me, more than half can be hidden with no problems at all, meaning the character shapes that we use are >50% redundant as well, at least when combined with the redundancy in words themselves (because an h of which the top is hidden look like an n, but the word probably only makes sense with one of the two).