Almost nothing in the web platform does this. If we need to do it somewhere it would be better if the caller handled it on its own. The default of NFCNormalize::Yes seems a little worrisome.
This will be easier to investigate with bug 254909 out of the way.
This is a longstanding difference, where we want to produce text in the same from as it normally happens on Windows, even if the source is decomposed. Basically, we want the same user actions to produce the same content on macOS and on Windows, even if this means that underlying web platform APIs behave differently. That's not great, and at some point, we need to perform normalization as text enters WebKit, not when it's sent to servers.
<rdar://problem/107805832>