And then of course, there's the caveat of you having to be able to reach a "significant target area". You have to jump through so many hoops to get a backstab I tended to ignore the ability was even on my character sheet. Not to mention the need for a creature to have a "definable back", which you wouldn't think would be an issue, but the rules specifically exclude Beholders, so, yeah.Yeah I was talking about MC or dual classed thief but even a longsword can work.
Dragons might be a bad example but everything else is available.
Remember loot was a lot more abundancet in older adventures along with things like Gauntlets of Ogre Power.
Magic longswords were fairly common along with exceptional strength one way or another (read 2E strength spell).
You're still dealing 1d12 probably with modifiers in an edition where outside Dragons and stronger giants hit points are reasonably low.
If I wanted to run an Assassin's Creed type D&D game I would bother with modern D&D. Castles and Crusades supports that idea better than AD&D even.
2E still best toolbox edition espicially for oddball games due to settings and splat books. Want a stone age game easy, want a Napoleonic War themed campaign that's an option.
It all depends on how permissive your DM is. But by the rules, backstab is hard, and worse, if you don't kill the monster, you're now at ground zero for it's reprisal.