You don't even need to search for older texts! The New Yorker actually uses the diaresis in their print edition. It's certainly a bit anachronistic, but I do think English spelling can be a bit obtuse to say the least, so the orthographic hint can sometimes be useful.
https://www.newyorker.com/culture/culture-desk/the-curse-of-...
https://www.arrantpedantry.com/2020/03/24/umlauts-diaereses-...
Although I'm a native french speaker TIL, thanks to you, that it's role was to inform that the letters are to be pronounced separately.
The diaeresis is very important in French (we call it tréma). Without this the word naïve instead of "na-iv" would sound "nev". The first name Loïc instead of "lo-ik" would be "looak" (it is even hard to say it loud for me). And many others.
Fortunately we still have the word for waters which is eaux and that we pronounce "o"
So we have several words that are pronounced "unexpectedly" (What You Read Is Not What You Pronounce). An example would be the name if the physicist Louis de Broglie, pronounced [dəbʁɔj]. OK, that's a family name so there is some history behind.
Then we have words that different people pronounce differently: ananas (pineapple) - some pronounce the "s", some do not. Or the village Avoriaz, same but for the "z". Or Chamonix.
But now that I think about this, what you are saying is indeed a thing. My wife's primary languages do not include French and she did not have, I think, major issues when choosing a pronunciation (she had Himalayas of other problems, though).
As someone who is only fluent in English but tried to study French for a couple weeks before visting Paris many years ago... Spoken French has institutionalized mumbling; what I read is you're supposed to form the final consonants of words with your mouth, but not voice them... Which ends up in practice with the consonants being pretty much voiced if the next word starts with a vowel and not if not... But I'm not surprised there's some variance.
The best written reference I could find (which sadly, I can't find again) was from a Hong Kong university where the writers seemed to have learned both English and French analytically rather than natively, so they would explain the rule for French and compare with the rule(s) for English and expose me to the rules I unconciously followed without knowing they existed.
Studying written and spoken French simultaneously was pretty helpful for me, otherwise it was too hard to fill in the ending letters of many words as needed.
It’s diaeresis. (Not sure if ‘ae’ can be replaced by the umlaut here.)
If diaeresis is too hard a word to deal with, we can just call it trema.
still, neither are diuresis, which nobody wants
Thank you for enlightening me.