Is This an Emoji?

The end of The article describes how 1 grapheme is a mostly good solution except that not all emoji work with the JS used to validate that, and the JS to validate it isn’t available in Firefox.

I think the point is that there’s no need to go to such lengths to enforce it. They build the picker UI so most users will just pick an emoji using that. Sometimes it’s important to limit degrees of freedom with strict validation but in this case I’d wonder if a non emoji would really cause a problem.

I think that's all fine, but implementing this restriction comes at a cost of maintainability. I'd say a much more pragmatic solution would be to allow a maximum of a few characters on the backend (say 10, because unicode) and then restrict the input on the front-end. If someone is willing enough to put other characters in that field by submitting the API request themselves, so be it - but it's also completely reasonable to turn around and tell them the reason their views don't look correct is because they submitted invalid data.

The reality is these users will be very few and far between, and as long as it doesn't pose a security risk there are very few benefits to strongly enforcing this limitation.

"what's an emoji" will also be different depending on the users, and while a service provider can just stand their ground on the whatever definition they want, that brings undue stress for the user.

For instance try explaining to a user why :heart: is ok but not ♥

I am not sure of the correct terminology but AFAIK you should be able to limit to display characters, not unicode codepoints.

99% of the time you might be right but in this case there is literally no benefit to the work described in the article.