Back in the AD&D, many spells had casting times in segments and you would as that number to your initiative. You would start casting on your unmodified initiative and the spell would go off on the modified initiative. During that time, if the caster was struck by an attack or fails a saving throw, the spell fails.
(Pulls out grognard beard, duct-tape-covered DMG of prior edition and a wizard robe)
All these kids complaining about counterspell have no idea how good they have it!
In 2e casting was a "no movement" action and you didn't get Dexterity added AC for that period and the spell was always lost if anyone so much as sneezed on a caster!
3e started getting all soft and mealy mouthed. Dang hippies! Spellcasting triggered AoOs, forcing a concentration check with a DC of 10+Spell level+damage dealt. Meaning 1hp of damage dealt while casting a 9th level spell required a DC20 check. Why, a caster might actually get to complete a spell when hit! Effete nonsense!
That next edition was a lot different with powers and such but those could also generate AoOs. It's like that thing between Highlander and Highlander 3, we don't like to acknowledge it existed.
5e has no AoOs, casters keep their Dexterity to AC while casting AND can move, and even a held action to attack during spellcasting do3s nothing! Only this here Counterspell does diddly or squat and just a couple classes can get it! What's with kids these days, expecting to get spells off reliably?!?
In my day, you wanted a villain to be a solo spellcaster, you pulled out a Gish so when their spells ultimately failed they could still beat people with a giant silver sword!
(Wanders off, muttering)