We never fixed it with rules, we fixed balance by giving the party magic items that addressed specific characters weaknesses. Pure thieves gained levels quicker, give them items to help them stealthier and survive, issue resolved.When we played 2e we always wanted a thief, but never wanted a pure thief. Fighters can specialize if single classed. Wizards can specialize if single classed. A cleric or druid at least gets faster access to high level spells.
Is a single classed thief the option you just pretend doesn't exist? Or is it for someone that specifically wants a weaker character?
If you think fighter vs fighter/thief vs thief isn't balanced then how would you fix it in 2e rules?
Its just example, maybe thats something I would have given a different class, its been too long. The point is look at the campaign, the other characters and provide a magic item that is obvious for a particular person (e.g. halfling sized shoes for the thief) and make sure the item benefits them and shores up their weaknesses so that each character feels useful.The only problem with that, Cavetroll, is by giving a Thief character a magic item that grants a 95-99% chance to Hide in Shadows (note that Thieves themselves cannot claim a 96% chance or higher to Hide, no matter how many points they put into the ability), you are just saying "scratch Hide in Shadows off your sheet, and I hope you didn't waste discretionary points on it".
Yeah but when we played 2e there was no need to give a thief anything at low levels, they were just cool, thats why people picked them, not because of their combat ability, that was up to the fighters to front line. The thief was sneaking around backstabbing people or maybe looking for treasure that he can pocket for himself. The whole "every class needs to be balance in combat" didn't exist, the fight was a team responsibility.Basically, if a 1st level Thief can't do anything, because he has a less than 50% chance to succeed, maybe there is something that should be done at a class level before saying "oh don't worry, pal, you kill a couple orcs, and wow, would you look at that, it's leather armor +2!".

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.