I don’t know how it started, but sometime after WordPress switched to Slack in 2014 the norms for checking in to a dev chat switched to emoji. Many people 👋, some use slack specific emoji like the bowtie, and others change it up on a regular basis.
Like many traditions, this started completely organically and at this point it’s so normal, no one bats an 👁.
I feel like It’s important to have traditions in open source software, but it’s also important to make them easy to pick up. It helps new contributors feel a part of the process. If the traditions are too hard to pick up on, then you risk creating an us vs. them problem. Thankfully, emoji’s to show you are present is one that people can pick up on right away and immediately feel a part of the WordPress team.