Whether it's a specific rule, or an unintended consequence of a new patch, the result is the same. People that got achievements or items before a certain point, get to use and keep them. Anyone wanting to get them after that point can not.
It's the same with grandfathered phone or broadband plans. It's not as though the companies specifically say that people can't get them, they simply no longer offer them. People already on those plans can generally remain on them though.
You must understand that you're the only one that thinks this way, and that, if by no other reason, in this context, the language has evolved to mean exactly what everyone (except you) uses it to mean.
This isn't something like people confusing then and than, or international spelling where both may be considered acceptable. It's the same as the word twink. It's most common usage within this game would be obvious to anyone. It also has other meanings, but it would be fairly simple to understand what it has evolved to mean, even if I personally do not like or use the word.