Nope. For example, any issue of the New Yorker is replete with uses of diaresis (see eg https://www.newyorker.com/culture/culture-desk/the-curse-of-...), and the OED does list fiancé; née and naïve are also pretty common; I was taught to spell words that had ligatures (eg hæmatology, œsophagus) at school...though I'm ancient. Wikipedia has more examples of this kind of orthography https://en.wikipedia.org/wiki/English_terms_with_diacritical... ... (yes I know this is 'just' wikipedia, but it links to more references on the topic)