Offside was introduced to the Game in the early 18th Century and has never been repealed. However, subsequent refinements to the rules meant that by the time of the famous Chalk Farm meetings of the early 80's it was virtually impossible to encounter an Offside situation once play had started so long as everybody plays by the rules (the situation where one uses Manor House to escape from a Beckerage loop is an obvious exception, although the Beckerage technique is disfavoured these days). The realignment of the zone boundaries in Holland Park 2000 meant that Offside once again became a consideration; although it can still be avoided with careful play.
The true reason that the Offside Rule has been kept in the ruleset all this time - indeed, the major reason for its introduction - is to limit the choice of opening moves. Without it, it would in many situations be legal to declare MC as the very first move of the game - which, as one commentator famously said, would sort of miss the point.