I thought it would be obvious because of the article headline, but email reactions. It's undeniable that emoji are useful for communication, I'm just not convinced that this particular interaction with an email is anything that anyone asked for or needs.
The only use case I can imagine would be for school/work accounts, but this feature isn't supported for those types of accounts yet. I'd assume that's because it's not yet integrated into the Office 365 platform.
The question remains: who outside of a corporate environment needs this? Maybe large families who communicate through chain emails? I honestly don't know anyone who uses email to have group chats anymore, but I suppose those people must exist. Just seems like it would be a small number.