This is what I do too. Basically, we are deflecting the awkwardness and/or blame to the software.

Or just "Hello, $reason_for_this_message"

That's less friendly.

Your director appreciates brevity over friendship.

No he doesn't.

Replace "Hello, $reason_for_this_message" with "Hello, $acceptable_level_of_polite_preamble; $reason_for_this_message", then.

I don't get what problem do all of you have with me saying "Hello Richie" to my colleagues.

Assuming the other end has filled in what Slack calls Display Name (which I interpret as "preferred name", is heavily implied by Slack's UI, and I think most people will too) … then you've addressed them as they prefer. (And if not, they can change it, and it instantly updates!)

I mean otherwise it's like literally any other communication medium, and you'll have to either take a culturally-appropriate respectful best effort guess at a preferred address, or just ask.

Or 'Hello Comrade Software Engineer/{x}' if you want to make it extra awkward.

I find that annoying when colleagues do that because it double notifies me, once for the message and once for the @