Now I'm thinking. As someone who's started businesses before and hired people - am I not empathizing enough with the employer? What would I do in this situation? I guess the answer to that is simple: first and foremost, I'd abide by my original agreement to end our contract in 2 weeks and not today. That would have side-stepped all of these concerns. Second, not having done that, not posted as me in Slack, indicating a blase lack of concern for my coworkers as I exited. "Thanks" isn't harmful, but it isn't helpful either. I had an opportunity to leave one last impression, and they squandered it with "Thanks". So, to all those saying there is no material harm: I ask, how much value does a person's last impression of you have? How much value does "Thanks" squander in that moment? I don't think that number is zero.