something that does meaningfully affect readability is WRITING IN ALL CAPS. you're almost guaranteed to have encountered the visually distinct shapes of lowercased words thousands more times than you've encountered the shapes of their uppercased counterparts

It makes visually scanning the sentences harder, capital letter is bigger than period. I mean, sure, it is readable, but I do prefer regular text, you know, like in a book or newspaper.

Deliberately making sentences harder to parse (by, e.g., bad grammar, bad spelling, lack of capitalization, lack of vowels, leet speak) is annoying.

Non-capitalization may not seem like a huge decrease in readability, but every little bit counts.