[int_19h]

https://en.wikipedia.org/wiki/Product_activation#Microsoft

Win2K was the first one for which I've heard about it personally, probably because that's when it started to show up en masse in developed markets.

[Hublium]

Thank you, was not aware of that. There was no mention on this Wikipedia article that I used to double-check:

https://en.wikipedia.org/wiki/Microsoft_Product_Activation

Let's rephrase @endgame's comment as "Windows XP and Office XP were the first versions of their respective series where product activation was mandatory on all retail editions." Point still stands I guess.