I think it would be very unlikely browsers would use a jit engine for xslt. They are removing it because they are afraid of the security footprint. A JIT engine would make that footprint much worse.
JS is already required to have a XML DOM parser, an universal XSLT engine in JS should be a low-effort web to continue supporting XSLT, as for performance the transform could probably be eval'ed and cached to JS snippets so that they in turn become JIT-compiled.
Somehow they have an issue with supporting, fixing (and updating to latest version) this particular one. Possibly because it doesn't result in promotions.
overall risk = new inherent risk / (architecture * security reputation * ongoing maintenance investment)
Even without arguing over whether JIT engines are inherently risky or add much risk given the modern computing environment is full of them, from graphics to Javascript.