Data consumers can and should be able to expect such values to be plain text. Requiring them to filter anything out that might get in the way is making the whole process unnecessarily complex, rendering the **structured** data of OSM useless.

I don't care what's e.g. in a description value, it's a freetext field, use emojis there as you want. But almost anything else is structured data which consumers have to be able to rely on. Never ever before have I seen Emojis anywhere in OSM. Furthermore, not every device/font displaying that data actually supports emojis. Yes, it's Unicode, but that doesn't mean every device and font can actually display it.

So, please: Don't use emojis where they don't belong.