Color management is just a must with nowadays wide gamut monitors. (More over Adobe Lightroom does not even allow you NOT using Color Management against the Monitor Profile). But Soft Proofing would be also welcomed and I suppose it's not that difficult to add when you already have Color Management implementation.
(Basically it's adding another in-the-middle color transformation: Picture -> Soft Proofed destination -> Monitor).
Which could be more difficult is its activation. While Monitor color management is fine as a Setting, (you don't turn it on/off continuously), you should be able to turn on/off the middle Soft Proofing step on the fly, (in order to compare how your soft proofed image looks against your original one). Therefore it should have a right menu entry access and a keyboard shortcut. (The Soft Proofing parameters, I mean the the color profile and the out-of-gamut way of rendering, are not changed so often so they can go as Settings). Maybe XnView is not currently capable of doing that on-the-fly color management change.
And as extra tools for both soft proofing and just color managed images the following masks:
- Showing the out of gamut pixels from the image against the Monitor.
- Showing the out of gamut pixels from the image against the soft proofed destination.
- Showing the pixels of the soft proofed image which remain out of gamut against the Monitor.