If the Thief were written with that assumption, I'd be happier about it. But the books never reflected that concept, and in fact, if you read things like the Mountaineering Non-Weapon Proficiency, you can see, by 2e, at least, climbing things if you weren't a Thief or Bard was intended to be HARD
I haven't read the Dungeoneer's Survival Guide in a long time, so I don't recall how difficult climbing was for non-Thieves in 1e. I do recall reading discussions that the Thief was badly received because the existence of their abilities just meant that things DM's decided characters could already do, were now class-locked.
Thieves need help, that's for sure, and if you take the approach that their abilities are "beyond normal", that might help them. Oddly, I don't care for the modern approach, which is that "everyone can do this stuff if trained, it's just the Thief gets more skills/better bonuses".
Mostly because that creates issues when you try to design encounters: "Ok, so everyone is sneaking up to the fortress, that's a DC 15 Stealth check." Players roll dice, and the Rogue player is like "what, I have a +17 with Advantage."
Which means you can't possibly use anything that could challenge the Rogue as a "party challenge", so either the Rogue always succeeds, or the rest of the party always fails.