> Kitty and Ghostty are the only terminals that correctly support Variation Selector 15,

Really? Because IIRC they "support" it by treating the preceding emoji character as being Narrow instead of Wide. This is completely unsupported by anything in the Unicode standards: the codepoint sequences that affect the East Asian Width of their first codepoints are explicitly enumerated, and none of them have emojis and/or VS-15 in it. So no, it's not "correctly" supported. Emojis are Wide (terminal width 2) with or without VS-15/VS-16, and if fonts have textual renditions of them that are only 1 cell wide, well, that's the fonts' problems, just as fonts that have e.g. glyphs for Playing Cards block with visible width of 1.5 (I am looking at you, Google Noto) are wrong too.

Agreed. The only thing that VS-15 is supposed to do is give the emoji a "text presentation", which the spec suggests as "black & white". And every example they show of a text presentation is exactly the same width as the emoji presentation.

Changing a character's width from wide to narrow after it has already been output is fraught with problems for a terminal. Imagine trying to write a "narrow" text presentation emoji in the bottom right corner of the screen. You'd think it should fit, but the emoji is received before the VS-15 selector, and that doesn't fit, so the terminal is forced to wrap the text, triggering a scroll of the entire page. By the time the VS-15 arrives, there's no way to undo all of that.

And for another example, try using IRM (insert replace mode) to insert a text presentation emoji in the middle of some existing text. If it was really narrow, you'd expect it to insert enough space for just one character, but it actually inserts two spaces, only occupying one of them, and then leaving an unexpected gap. And the more of these "narrow" characters you insert, the bigger the gap becomes.

VS-16 changing a text presentation character from narrow to wide doesn't share these problems. And that behaviour is supported by the spec text, which says that emoji should generally have a square aspect ratio. And at one time the East Asian Width spec specifically mentioned VS-16 making narrow characters wide (but said nothing about VS-15 making wide characters narrow).