Sure, in the past there are examples - but in the present, it seems it is now a regular occurrence. Also, in the past, there may been good reasons. It wouldn't have been as easy for players to travel, or for scouts to travel to watch players. Sometimes, testing players out in a match would have been the best option. There weren't as many international matches played in those days either, so maybe they just counted more of the dubious cases.
For Slovakia this time - I don't think sending these new players to the UAE and making them play an international match was the best thing to do - why couldn't they just label it as Slovakia 'B' (which is what it was)? Or couldn't they test these players out in Slovakia against a club team? Slovenia did the sensible thing - they labelled their team a B team, because that's what it was. If other teams did the same, there would be no confusion.