This is going to sound strange, considering I spent bundles of energy defending PbtA / FitD / Ironsworn in the Apocalypse World thread, but I can kind-of, sort-of understand what FKR is aiming at, a bit . . . .
Our current game of Tiny Frontiers (Tiny D6 sci-fi in space) is probably pretty close to an "FKR" experience in a lot of ways.
If you read through the Tiny Frontiers rulebook, there's almost nothing there. The entire mechanic is basically, "Roll 1d6 if it's something you're terrible at or disadvantaged, roll 2d6 if it's 'normal' circumstances, or roll 3d6 if it's something you're good at / advantaged. As long as you get a 5 or 6 on any of the dice you roll, it's a success. A critical success is if you roll a 6 on any two dice (meaning you can't critically succeed on a disadvantaged roll)."
There's no other bonuses or mechanical modifiers to anything. Literally none.
There's a very small count of minor racial trait adjustments that give circumstantial "advantage" rolls, an absolute bare-bones hit point system, and maybe 10-15 pages of slightly more "layered" rules for damage, armor ablation, and a pretty nifty theater-of-the-mind space combat system that is actually tremendously more fun than it has any right to be.
But it 1000% falls into the "minimalist" camp of design. And though it's unstated in the rulebook, it quite obviously and heavily leans into the notion of "GM decides" as the primary loop of play.
So I recognize what FKR proponents are saying about fictional positioning being the pre-eminent determinant of 'what's possible' in the game. There's nothing in the rules that says what happens when you punch a hole in a space cruiser with an assault rifle, it's just assumed that we'll all sort of agree that rapid decompression is the likely result---and if we don't agree, the GM is supposed to just jump in and say, "This is how it is."
As a result, I can sort of picture what the FKR proponents are saying, where the first basic input of action declaration is, "Where are we in relation to the assumed fictional universe?" Because in our Tiny Frontiers game, so much is just assumed / hand waved / ignored until suddenly it's relevant, and someone has to decide whether it works or not. For reference, the basic campaign setting for our game is the universe of the video game
Deep Rock Galactic (which, apropos of nothing, is an awesome game in its own right).
But beyond saying "Okay, we're in
Deep Rock Galactic," there are zero governing principles in the game rules about who is allowed to introduce
what kinds of things into the fiction and
when they are allowed to do it. On absolutely no level would I consider it to be a "high agency" game.
And like
@pemerton has noted, Tiny Frontiers really only works as a "ruleset" when its immediate focus is on
tactical engagement --- it's highly gamist in that way. There's no rules, stated or implied, to introduce any kind of relationship / consequence / background milieu information through the players, other than the players just throwing things out there and seeing what sticks with the GM.
In a way, it feels like FKR is basically a complete surrender to the idea of "illusionism as principled play." I have no idea how our GM is really handling the inner workings of the fiction behind the scenes. What is determined as "meaningful" or "matters to the characters / matters to the fiction" is entirely behind an impenetrable "GM Third Wall."
If Tiny Frontiers leans heavily into the realm of FKR --- and everything I've read of FKR in this thread would seem to point that way --- I would find it difficult to see how FKR is viable for anything more than short-term play (mini-campaigns of 6-10 sessions), and only in circumstances where player-character actions are allowed to resonate within highly pre-framed areas of play that focus on tactical engagement.
Truthfully I was skeptical as to whether Tiny Frontiers had enough "bones" to really be fun, and I was wary of the GM, having had to suffer through a largely "setting tourist" campaign of Savage Worlds: Shaintar with him previously.
But strangely, and in spite of my misgivings, Tiny Frontiers has been a lot of fun, probably because our game has largely stayed within that fairly limited framework ("We're space dwarves, mining and hunting alien artifacts, swilling beer, and blowing stuff up!"). It's not what I'd choose to run/play all the time, but it's certainly been a worthy 4-month diversion from other games we've played.
TL;DR --- Imagine stripping away the entire D&D 5e ruleset until all that's left is rolling normally, advantaged, or disadvantaged on a static TN of 9 for all rolls. Characters get one, maybe two racial traits / feats that gives circumstantial advantage rolls. All damage is normalized as 2, 3, or 4 points per roll (and no others). Your amazing, hand-picked, "highly trusted" GM has final say in all other fictional positioning arbitration. Oh, and you occasionally get to play a fun "pirate ship" sailing mini-game, if you want. If you can picture the sort of game that would arise from that core conceit, you're getting pretty close to the realm of FKR.