@Zinon I believe that was something in the official rule book me and my friends also stacked +2 and +4 but at one point we read the rule book and we found out you actually can’t do that but it’s a lot less fun, and more annoying, so we didn’t follow it.(this was over 10 years ago so idk what the physical deck rule book are saying now)
I mean nobody really reads the rule books so naturally people would go stack +2 and +4 because the basic rule of uno of being able to stack something with the same number or the same color but by the rule technecally +2 and +4 are not number cards so you can’t stack them just like how you can’t stack stop reverse and 4 colors they are in the same category.
By the rule book too if Remember correctly you can’t Uno or win the game if you uno with a special card (+2,+4,stop etc) another common rule that people don’t follow. You are forced to draw a card.
Board and card games has always change rules anyway we just manage to see it change in our current generation just like how chess used to have more piece and different board.