...and so on and so on. But, I think this was gameplay emergent from a DM-ing/adventure style, rather than the rules, as such.
That's a bit of a feedback loop between the rules a DM has available to them and the
I was reading a blog recently that talked about (Grognardia? Alexandrian? Rients?) the fact that White Box D&D had very few rules for dungeon-crawling, but one of those rules was for torches being blown out by gusts of wind. Early D&D modules? Gusts of wind all over the place.
Those rules get de-emphasized with AD&D (as a bunch of other dungeon-crawling rules are added) and the gusts of wind disappear from the adventure modules shortly thereafter.
Pre 2E, IIRC, the thieves' "Climb Walls" was just that, incidentally - not "Climb Sheer surfaces" at all. Which left a weird "grey zone" with easier climbs that, reasonably, anyone ought to have a chance at but no system existed for.
Somewhat true, but mostly not.
OD&D, Supplement 1: "- climb nearly sheer surfaces, upwards or downwards" (So here's it's explicitly sheer surfaces.)
Holmes 1977: "climb sheer surfaces" (Maintains OD&D's definition.)
Moldvay 1981: In the table it's "climb sheer surfaces". In the text it's "climb steep surfaces". (So here it becomes muddied.)
BECMI 1983: "...any steep surfaces, such as sheer cliffs, walls, and so forth." (And here the muddiness gets cemented into the rules. This is maintained in the 1991 Rules Cyclopedia with slightly different verbiage.)
So you can see the "sheer" verbiage is actually the earliest incarnation of the rules. It then evolves to include non-sheer surfaces, but still seems to stay mostly focused on sheer surfaces through the Basic line. (And this is emphasized because BECMI included explicit rules for non-thief climbing.)
Meanwhile, over in AD&D...
AD&D, PHB: "Ascending and descending vertical surfaces is the ability of the thief to climb up and down walls. It assumes that the surface is coarse and offers ledges and cracks for toe and hand holds." (Explicitly
not sheer surfaces, creating that weird grey zone you're talking about.)
AD&D2, PHB: "Although everyone can climb rocky cliffs and steep slopes, the thief is far superior to others in this ability. Not only does he have a better climbing percentage than other characters, he can also climb most surfaces without tools, ropes, or devices. Only the thief can climb smooth
and very smooth surfaces without climbing gear." (As with BECMI, AD&D2 includes rules for non-thief climbing. In fact, these rules in general appear to be heavily influenced by BECMI while eschewing AD&D1's approach entirely.)
You'll notice, however, that the system was
never just "climb walls".