If you substitute the phrase ‘flat character growth’ for ‘bounded accuracy’ then I would suggest there are many games which will scratch this itch, including a swathe of OSR games and modern fantasy games. Even a game like GURPS has a pretty flat power growth structure in my experience (though there are some ‘big’ advantages you could buy that are game changing, like Trained by a Master). Main branch (A)D&D is more the outlier in terms of character growth, in my experience.
I think you underestimate the number of games which exceed the growth rate of AD&D... literally multiple hundreds of variants on 3E alone, and a hundred or more of AD&D or BX/BECMI just in the last 25 years. Not to mention the several hundred D&D knock-offs with a few changes and/or better wording from before the OGL.
There are probably over a thousand D&D variants out there... and the #2 game in the world most of the last 10 years is a D&D variant, too: Pathfinder. And there's A5E, Draw Steel, Daggerheart, and a half dozen more in reaction to WotC asininity over the last 24 months.
And that's ignoring games like Rolemaster, which started as a set of optional combat mechanics for use in AD&D, but became its own thing, and has, potentially, even faster advancement...
Or, Palladium, where several dozen various Rifts classes (OCC/RCC) have massive increases due to specials which come in at mid levels...
Or, the Arcanum, which increases skills just like Palladium, but also adds more more often, and also increases casting or fighting prowess faster, but has only slightly lower HP advancement, and similar speed of leveling.
The D&D tree is more than half of what's played. The D&D tree is well more than half of what's sold. And most of the D&D tree is faster than any TSR D&D RAW. Excepting maybe a bunch of the OSR games... but even there....
Simply put, "Not part of the D&D main tree" = outlier in the industry, and = Outlier in terms of what's being played in Roll20 or other major VTTs. Which implies strongly that it also equals oultier overall, except perhaps in the total number of games ever released. Especially since almost every D&D variant advances faster than AD&D RAW... most often by simply omitting the nastiness that is the training and play quality requirements in AD&D... (buried in the DMG. If adhered to by the GM literally, they can halve the rate of, or even permanently prevent, advancement...)
Even many of the not-D&D games advance in power quickly - D6 Star Wars can rapidly get out of control given the 5-10 SP per session. Especially with Jedi - whose force abilities can exponentially grow power.