Oh I agree, the DM -should- be working with the Thief. Unfortunately, the guidance in AD&D seems to be the very opposite.
While not relevant to the Thief specifically, we have Gary actively saying that you should NEVER tell Assassin players that they have an extra class ability (poison use) unless they ask specifically (hey can assassins use poisons?). His rants about player agency inform the DM that the players need to work for everything, and you should make things as hard as possible for them.
Ha! Funny story, this actually just happened in my ''Team Evil' 1e game - our 4th level Assassin queried about learning poisons. (completely oblivious to this reference in the books)
Gary's DM vs Player rhetoric aside... (I certainly don't subscribe to actively opposing players) I do think players should work for it, but sometimes circumstance and mechanics will make things easy, and I won't take that away when it happens. Nor will I make it easy when things get hard. Neutral arbiter and all that...
On the note of the 1e Assassin - they actually get backstab at first level, but no thieving skills until 3rd level. If one is running the 'Must succeed in moving silently or hiding in shadows' metric to backstab, then assassins having the ability at 1st level is absolutely pointless.
And sometimes, that's baked into the rules- for example, Monk open hand damage doesn't work on creatures of a certain height!
It's the stun/kill effect that doesn't work - the damage still applies. I'm okay with this rule, I don't see it as a negative as it's a defined parameter, not left to on the spot arbitrary decisions by the DM. Monks can be brutally deadly in 1e even with it. (Especially if you apply the surprise metric to Thief skills as I have been outlining where in 1e surprise can grant multiple actions if you get extra segments).
That's the crux of the matter here - a systemic limitation is fine, when it's clearly laid out so we know how to apply it. But when the DM comes in and takes away abilities for arbitrary reasons, that's when the trouble begins.
Thieving skills are overwritten with so many addendums that they cause debate... but if you go back to how what is printed can be applied through a clear system of procedure, the problem clears right up.
Did my stealth roll succeed? If yes, gain bonus to surprise. If no, surprise chances normal.
Did I achieve surprise? If yes, my rear attack is a backstab. If no, then my rear attack is just normal.
Did my pick pockets check succeed? If yes, then I snagged something. If no, check roll against HD of opponent. Was roll high enough to be caught? If yes, then DM will adjudicate response. If no, then *phew!
You get the idea.
2e doesn't have anti-player rhetoric, but the rules still reflect that things should be hard. Some of the things I noted about the Thieving Abilities just boggle the mind. If I Hide in Shadows, infravision cannot detect me. But I can't Hide in Shadows in pure darkness, so infravision can detect me easily before I can reach an area with some kind of illumination?
There is a logic behind it, though. Infravision is spoiled by a light source, hence why hiding in shadows will beat it. But in total darkness, everyone except those with infravision are blind... so having infravision in total darkness will allow you to see the person attempting to hide.
If you're in complete darkness with others who are blind, then hiding is irrelevant as you can't be seen anyway. Moving silently, however, is not irrelevant.
And the truly tragic part about this is that backstab isn't really even all that good! The damage multiplier is to the weapon damage dice alone! Now the Thief does have a few weapons that can really take advantage of this (a broadsword does 2d4, so 4d4, 6d4, and so on isn't bad). Elves will probably use longswords which are a little worse at 1d8.
Halflings can use these weapons in two hands and it's not like Thieves use shields or anything.
Presumably a Fighter/Thief could use a Greatsword or something crazy like the Longspear from the Complete Fighter's Handbook.
But even then, this isn't guaranteed death, even though it can be a lot of damage.
It's good enough, especially used in conjunction with allies. Certainly effective enough to warrant the effort at any rate. It speaks to how thieves approach combat, especially against enemies who are distracted by your team. Backstab, duck out of combat when enemies are distracted, hide for a bit, and try again.
I have always ruled that backstab must be done with a thief class melee weapon, regardless of your class combinations, but then I also go with the PHB note (in my 1e games that is) that backstab can be attempted so long as you're in a rear flank, surprise simply gives you +4 to the attack.
When I ran 2e, I allowed any weapon (within reason, no ballistas.

), but it was btb in that you needed surprise.
You can see this with a lot of the early 3e discussions about the Rogue's Sneak Attack, where DM's, used to older rules, flat out rejected the notion a Rogue could Sneak Attack with a greatsword or a scythe.
Then in 4e and 5e, what weapons you could Sneak Attack with was restricted, so that while 5e Rogues can use longswords, they can't use them to Sneak Attack.
I know that this is an example of DM's stepping beyond the rules, but this is the kind of environment the Thief lived in- his abilities were hard to use, had glaring flaws, and the DM was sometimes encouraged to make the Thief fail.
I'm certainly not saying it's not a thing that happens... I have seen it. Felt it. Been there done that. In 36 years I've certainly been THAT DM... I've been the player on the receiving end of THAT DM... but that's one of the reasons I took a hard look at what the rules lead to if you just approach it systemically, and came to conclusions that I have.
I've been using it in practice for many years now, and it's effective. My niece played a thief once, and her stats were super average. Dex of 11, nothing else higher than 10. (Yes, I'm a DM who makes people keep their rolls come what may, but that's another debate thread...

)
Anyway, that character was a howl because her abilities were so low that almost every effort failed at first. But even so, even with failed checks she still had the base 1-2 on d6 surprise chances. (this is 1e, so d6 not d10). That's a 1 in 3 chance of still achieving surprise, and she did so often enough that even her low thieving scores didn't discourage her from actually making attempts.
I can't speak to 4e, as I've never wanted or needed to play it. I do dabble in 5e, and while I think sneak attack is a little on the permissive side I wouldn't nerf it or anything - it certainly works within the context of how 5e plays. But the system is there, and it's quite clean and easy to grasp with no open ended clauses for DMs to to cancel the move.