I didn't take it any particular way besides maybe a hint of informality. Regardless it definitely threw off my reading in an unpleasant way. Anecdata point for the poster maybe. I would read again.

I don't read it as informal. Childish or lazy perhaps, at best.

When you intend it as "inviting informality", you're implicitly doing this because "formality" is too much effort. The thing is you're not inviting, but rather demanding. You have decided the conversation is informal and low effort, and that's how you'll treat it, without considering the person you're communicating with.

This is of course all a lot of strong statements and these things don't matter as much as this sounds. I don't feel _that_ strongly about these things, but the lowercase thing always strikes me as just plain weird, and deconstructing why I feel that way this is where I get to.

there is a very big difference between starting an ephemeral instant message without capitalization, and lacking capitalization throughout a more "persistent" long-term medium

It's not derived from that. Most people don't use IRC anymore and there are basically no open chat platforms left. Most people now use phones to chat, where the keyboard automatically changes case for you after finishing a sentence.

So typing in all lowercase is a choice. I think it's a way to signal "I don't follow norms". It signals membership in a group.