Less common than it used to be, but still happens. Mostly when writing in a hurry (of course without auto-correction turned on) or interacting with older systems and applications. Also, it is so rare to use Polish letters in a URL, that if you were to dictate one based on a Polish word(s) most people would simply spell it without them (ł → l, ą → a, etc.).

By the way, never try to use Polish letters in a Windows username. It works, but every once in a while something third-party or rarely used breaks unexpectedly.