RuneScape Classic is, basically, a snapshot of RuneScape since it initially released in 2001. That initial version has observed two big improvements in the years since, as well as (slightly confusingly) the coming of Old School RuneScape - a variant of the RS gold game more or less as it was at 2007.
As RuneScape has evolved and grown however, RuneScape Classic has continued to live on in a dusty corner of Jagex's servers. It's no longer updated or formally supported by the programmer, but access is still allowed to certain players.
In its article announcing Buy RuneScape gold Classic's closed, Jagex explained that"It's been wonderful to see such dedication amongst those of you who've kept playing RuneScape Classic over the past couple of years [...] However, it is not all fun and games.
"With improvements in technology helping to further encourage both RuneScape and Old School RuneScape, our resources are no longer compatible with Vintage. This is particularly a problem with our network security and macro detection applications. The game is now easily abused with the use of 3rd party macro tools, and botting has become a growing issue."