I might have terrible grammar and everyone knows it except for me Do you notice people misunderstanding you? That’s how you’d know.

The case where people know about a grammar issue but still understand the writer is a counterexample to your claim.

As someone who was raised Rural and will drop a lot of dialect into speech, I've seen how it colours the response and taints the message. I've seen where people spot a bad habit and just let it go. People know.

The more one learns about structure and form, the more one notices these gaffes; and not only does it change how the message was received, but it can also be one-sided. For instance: people who say "some emails" don't realise how discordant it sounds to people who don't say "some mails". People who say "some email" I think aren't generally noticed by people who say "the mails".

What you do with what you notice? That's on you. But in a world where first impressions are actually a thing, it'll be noticed.