I actually like this form of the name better than the somewhat more familiar Isolde and Isolda. I like a lot of names that start with letters you don't see very often, and I think it's kind of neat to see a Y name that usually starts with an I, like Yseult or Ysabelle.
According to this source: https://en.m.wiktionary.org/wiki/Yseult#French [noted -ed]