The Unicode Emoji List is revised more or less annually. This means that
releasing software that validates this header against the
contents of UTS #51 is liable to fall out of date.
The entire Unicode standard is revised more or less annually This year
Unicode 13 added over 5000 characters to Unicode 12. If you send a new
emoji or any of those other 5000 characters you run the risk that
someone with last year's software will see a little box. That's life.
The problem here is that some implementors like to validate data at layers
were its validity is (probably) irrelevant to its intended
"Don't do that".
If they haven't already figured out that it's a fool's errand to
validate Unicode beyond broad rules like don't allow surrogates in
UTF-8, they're not going to figure it out for this tiny hack.
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