Thanks for the correction. To be honest that didn't occur to me - as I was growing up it was very much lumped in with "other old fashioned stuff" that was dismissed as conservative bokmål and I've missed the Norse connection.
I know it's still seeing some use, but the steady reduction in use was very noticeable already during my school years in the 80's and 90's, and appears to have continued. Some use will still certainly persist for a long time.
The fun thing is we can easily quantify the relative rise and decline in written use at least by searching the national library (nb.no) for newspapers. I've only done the search for one set of numbers, so a major caveat that maybe there's large variance between different numbers, but a search for "fem og tredve" combined with "femogtredve"
* 2000-2024: 412 newspaper hits
* 1950-1999: 3112 (caution, different bucket sizes)
* 1900-1949: 4303 (before the reform)
(the "halfway point" og "fem og tretti" is also in use; 42 between 2000-2024, but never very widespread)
vs. "trettifem":
* 2000-2024: 2587
* 1950-1999: 5728
* 1900-1949: 69 (before the reform)
(You can break down the search results in finer chunks too, but I think this gives enough of an indication)
