Iajuitsu proficiency was a thing in the AD&D 1st Edition Oriental Adventures, but I don't it worked very well. While most classes were pretty good in that edition, I wasn't quite a fan of the Samurai in it. I feel like Paladin would be a better choice, unless of course you liked all of the titles and responsibilities Samurais got as they leveled up. Forced proficiencies you had to take by a certain level were a pain too. It's one of the classes later editions did better.
In 3.0, it was amazing. On it's own, it's not fantastic - Iaijutsu Focus was a separate skill which any class could take. Normally, it's used in dueling, which is a huge part of L5R. The main reason Kakita Duelist was a big focus was because in 3.0, there's not many ways to get damage outside of high Strength, two-handing and Power Attack shenannigins.
Kakita Duelist was the Crane Clan PrC, which in L5R, are basically the Anime Samurai. They're pretty, they focus on katanas and they loooove dueling. They focus on being fast. But in every edition of L5R I'm familiar with, they get the ability to use dueling rules -in the middle of a combat-, as otherwise, they'd be useless.
So, Kakita Duelist did a similar thing. It allowed you to use Weapon Finesse with katanas (which normally isn't possible), so you could drop Strength. It also allowed you to use Iaijutsu Focus in combat for massive damage, but additionally, as it's a skill and not an attribute, it could be increased every level (and Samurai, like Fighters, don't really have much in terms of skills to really bother with. So it was essentially a free damage boost every level, which could be further buffed with Skill Focus, etc.)
I was absolutely not making any reference at all to "Japanese steereh fordred one mirrion times!" by that post, merely making mention of actual Japanese combat techniques being focused on nimbleness and not strength. I wasn't doing tabletop gaming in 2005, so I thankfully missed all of that katana-related bullshit.
And I'm sure I've ranted about how much more useful Dex is to fighters in general than Strength, so its an easy way to min-max for both damage output and survivability. I shamelessly did exactly what you described with dropping strength to go max dex in a 5e game where the GM set his fantasy Japan at the end of the Warring States Period, so I got to use a gun along with the hilarity of a Finesse greatsword called a nodachi. (What's that, wear armor? Don't be ridiculous, dexterity lets me dodge bullets!) Of course the Monks got to use halberds naginata as monk weapons by default, so it wasn't that OP in game context. Only somewhat OP.
That's 5e, not 3.x, which is what I'm talking about. In 3.5, Weapon Finesse was essentially the Finesse tag, but it didn't apply Dex to damage - the main reason for it was for classes like Rogue, who didn't really use Strength for damage, and relied solely on Sneak Attack dice. But yeah, if you're going to throw 'Finesse greatswords' into your game, where the only upside to Strength is getting access to the best melee weapons in the game, yeah. You're gonna get -every- character using Dex and studded leather. =P
Has anyone ever managed to balance Str and Dex in 5e? I don't think there's any reason to build a Str fighter besides flavor. With a full 20 Dex build and studded leather armor you may lose 1 or 2 AC compared to a full plate, but the benefits way more than compensate for it.
I don't really play 5e, but every game I've seen or played in, everyone builds Dex rapier users, because there's no reason not to. Very specific characters can get away with high Strength builds (Fighter with Great Weapon Mastery). Dexterity/Agility/etc. has always had an issue in a lot of games as a godstat.