I'd say it can be written this way. Not that it is taught to be written so, or even encouraged, but everyone who reads it will understand what is meant. I do not believe this is a remnant from the Kurrent or Sütterlin, because there a bowed line denoted something completely different, as has already been pointed out.Two short vertical lines or even a wavy line denoted the Umlaut. Also these older scripts are not intelligeble to most younger persons, not even in German speaking areas. Instead this This question isn't really limited to the German language, and might even be off topic because of that, since exactly the same diacritic signs with the same function are found in many other languages. The line, (curved, wavy or straight) is a very common way of writing these diacritic signs signs in other languages using these. In Swedish we were taught to write the dots as a curved line in cursive. https://sv.wikipedia.org/wiki/Skrivstil#/media/File:Svssfb.jpg (Nowadays cursive handwiting isn't even taught anymore.) The same goes for Danish and Norwegian. I therefore think it might have more to do with a common trait than with a supposed link to older ways of writing. The fact that writing this way in German could cause a confusion, because of the old n/u distinction function, shouldmight make it less approprate though.