Adding any emoji as a reaction remains under review.
I am marking this as partially done to let you know a new reactions bar with Like, Heart, Laugh, Surprised, Sad, and Angry has begun rolling out to all users.
Thank you for your continued feedback. As many people have pointed out, this is available on mobile today. Support for replying to a specific message is on the backlog for desktop. I’ll let you know when we have progress to share. Thanks.
You can see this right above your text field when someone is typing. It will say “[Person’s name] is typing…”