What Lyonel said. I’m away from my references 🙁 , but see North American Icelandic.
The story is that Icelanders noticed the merger in the 1920s, stigmatised it as “fisherman’s language”, and got rid of it successfully (although the link says that the e/ö merger is still around). In North America, of course, no prescription, so flámæli has kept going, which what the linked book is about.
The importance of flámæli, which is why I keep mentioning it here and misspelling it, is that language change can be reversed through prescription, but you need special circumstances; and Icelandic is as “special” as it gets (small community, universal literacy). And given that the e/ö merger shows up in Reykjavik teens 60 years later… I guess the success was only temporary.