I think that somewhere in this answer lies a reason why Windows still doesn't support flag emoji. I don't count Microsoft Edge as "Windows" in this case, but as Chromium. Windows doesn't support flag emoji in its native text boxes, but it does support even colorized emoji.
But then again, flags seem to be not only Unicode-hard but post-Unicode-hard.
Flags are political. Microsoft has removed country borders from its products for political reasons, a post above says the flags rendering was excluded for the same reason.
> But then again, flags seem to be not only Unicode-hard but post-Unicode-hard.
Flags are not that hard, they're a very specific block combining in very predictable way. They're little more than ligatures. Family emoji are much harder.
Consider you have to split a string with 20 flags in sequence at a given offset. That's 40 codepoints with no readily discernible boundaries. To parse that you have to scan backwards to find the first non-flag codepoint. Otherwise you could split the middle of a flag pair. You also have to handle rendering invalid combinations as two glyphs and unpaired codes. For normal codepoints with combining characters you can scan forwards until you reach a non-combining character.
Flags are not that hard, they're a very specific block combining in very predictable way.
But before their introduction, you could decide if there's a grapheme cluster break between codepoints just by looking at the two codepoints in question. Now, you may need to parse a whole sequence of codepoints to see how flags pair up.
But then again, flags seem to be not only Unicode-hard but post-Unicode-hard.