Yep, add a layer of salt in winter and the plates are often completely, 100% not readable even if you're standing 2 feet from them in broad daylight, leaning over to try to discern the characters.

It's not legal. Your plates have to be legible. But much like many other portions of the Traffic Act, it's unenforced, so it's not really a "law" per se, more like a suggestion.