Reaction icons on Slack/Discord posts are actually useful. Adding a "+1", "agreed" or "done" react on a message is far less intrusive than a reply message to that effect, especially if you don't have threading (like IRC). You could even use them as an additional input/output signalling channel for automation or bots, if you were so inclined.
That aside, communication tools facilitate human to human communication first and foremost. In natural person to person communication, there are many additional information channels than just the content of your speech. There is your tone of voice, body language (however overrated that might seem), gesturing, scribbling a quick diagram on a napkin, laughter, winking, and whatever else. A good communication medium will strive to let you communicate as many of these things as possible, by analogy if needed. A reaction GIF can communicate a great deal about the non-tangibles of the other person's state of mind, and help you both understand their intent better, and know them better as a person.
If you want to keep your communication serious, establish channel rules that forbid certain types of expression to change the tone of discussion, or use a client that doesn't implement such features/lets you disable them. Don't limit it at the technology or protocol level.
That aside, communication tools facilitate human to human communication first and foremost. In natural person to person communication, there are many additional information channels than just the content of your speech. There is your tone of voice, body language (however overrated that might seem), gesturing, scribbling a quick diagram on a napkin, laughter, winking, and whatever else. A good communication medium will strive to let you communicate as many of these things as possible, by analogy if needed. A reaction GIF can communicate a great deal about the non-tangibles of the other person's state of mind, and help you both understand their intent better, and know them better as a person.
If you want to keep your communication serious, establish channel rules that forbid certain types of expression to change the tone of discussion, or use a client that doesn't implement such features/lets you disable them. Don't limit it at the technology or protocol level.