This feels like something that should be opt-in, not opt-out. It feels trivial to have all clients that support it send a header stating they do, and it is ridiculous that the default is to allow sending reacts to clients that don't support them
I don’t really care for the Outlook reactions and find them out of place, but this implementation doesn’t break anyone else. It’s also exactly how Apple implemented reactions being sent to SMS recipients.
Disclosure: I work at Microsoft.