To enable live reactions in Microsoft Teams, you can do the following:
In a meeting:
Click the Meeting controls button.
Click the React button.
Select the Allow reactions check box.
The issue I have with this answer is that this is not always user controlled - this was turned on by default at app level without my knowledge and happened on random calls. i know I didn't enable it myself as there is no way on earth I would ever want this. I also know that the IT administrator didn't enable it because that is also me.
Fortunately it happened in an internal meeting so I didn't look like an idiot in front of clients. On the other hand maybe it did, and the clients were too polite/horrified to say it.
For anyone else who finds themselves in this situation go to settings > appearance and accessibility > animation and click 'turn off animations'.