BoldItalic
First Post
If Lefty is average-to-good at mental tasks in general, then give him average Intelligence. That is the raison d'être of this whole "Intelligence score" mechanic. If Lefty does not have average Intelligence, then in gameplay he will not be average-to-good at mental tasks in general. Can Lofty perform [insert mental task here]? Intelligence check. -3 penalty. This isn't tyrannical DMing; this is fundamentally how the ability score rules work.
And it is simply not true that you cannot penalize Lefty's lore-based skill checks. Of course you can. You can opt not to make the rolls, or make them with disadvantage, on the basis that it wouldn't make sense for him to be able to do that. If I make a character who's terrified of water and doesn't know how to swim, I don't give him a very low Strength score and model his average strength by buying up the other Strength-based skills. I give him an average Strength score, because that's what Strength is there for, and then when the need for a swimming check arises, say "Sir Archibald the Dry doesn't know how to swim".
To paraphrase the introduction to the PHB: specific beats general. You don't have to try and use the rules for general lack of ability to model a specific lack of ability. It's not what they're there for, and it results in unintended and undesirable consequences.
That's an interesting concept - that we attach to a character sheet a list of tasks (things for which ability checks are made) that are to be made with disadvantage. It's not something suggested in the books, but it could certainly be done.
Rather than disadvantage, though, I would propose that those specific tasks are made at, say, -5 (which is the usual equivalent of disadvantage).
The problem with using disadvantage for this, is that it only reduces the probability that something can be done, it doesn't block it altogether. If we give Lefty Int 12 (+1), say, but with a blanket disadvantage to reading-related checks, he can still read a complex tome (Hard task, DC20) if he rolls high enough (1% chance). That doesn't seem right. He shouldn't be able to read it at all.
But I like your idea. Perhaps the simplest way to do it is to have a (short) list of tasks that the character cannot perform at all (swimming, lore and so on) and say: no roll involved for these tasks, automatic fail.
The flip side of that would be a (short) list of tasks flagged as automatic success. All mundane tasks already come into this category (characters automatically succeed at walking down the street, or eating a sandwich) so we can add extra ones that, for this character, are deemed trivial. I might say, for example. that Lefty can automatically open locks.
Perhaps it ought to be limited to a single handicap and a single "super skill". It's changing the game, though. I think some playtesting would be needed.
BTW, BoldItalic automatically succeeds in arguments on forums but he automatically fails at reading Japanese.