I wrote the below for our internal user documentation. I haven't seen this reflected in the official Microsoft stuff:
If you'd like to "Un-react" to a message, this cannot be done from the Classic Outlook Desktop application. You'll need to use Outlook on the web (outlook.office.com/owa), the Outlook mobile app, or New Outlook to remove your reaction to an email.
Outlook mobile app:
- Open the email from which you want to remove your reaction.
- Tap the smiley face icon to see the reaction options.
- Tap the reaction you left previously.
- The reaction will disappear from the email for you and the recipient of the reaction. It will also disappear from the user’s notification pane. If the user’s notification pane is open, it will disappear after the notification pane is closed and reopened.
New Outlook and Outlook on the web:
- Either open the email or view the email in the reading pane from which you want to remove your reaction.
- Tap the reaction you left previously.
- The reaction will disappear from the email for you and the recipient of the reaction. It will also disappear from the user’s notification pane. If the user’s notification pane is open, it will disappear after the notification pane is closed and reopened.