Hi,
yes, sadly UWP platform is just very carelessly put together and they simply did not include xml and some other features... amazing... so on UWP you can't work with XML when using Unity ( I am not sure about other dev platforms). Maybe 2017.2 and the 4.6 .net framework woudl allow for this I haven't checked.
Actually you can, I have heard that if you are ok with dlls and visual studios there are ways to include the missing dlls, but I admit my lack of understanding on this.
Bye,
Jean