Has Counterspell always been able to target any spell on the stack?
Dark Ritual was printed as a 'Mana Source' rather than an instant from Mirage to Urza's Saga. Back then, Counterspell was an interrupt, which was something of an 'instant-but-faster'. You could not counter mana sources like Dark Ritual because they were 'too fast', just like you can't counter a mana ability under today's rules. However, creatures (back then, 'Summon [creature type]' cards) have always been vulnerable to Counterspell and its ilk.

Some more information about how the rules worked back then can be found in the 2002 article 'Rules, Interrupted' by Ben Bleiweiss on wizards.com.