1) That setting still exists, it's just less prominent.
2) Subpixel AA is no longer the default; the default is grayscale, because it works well on all screens in all orientations.
Apparently it is supposed to be on for low DPI screens. I would count mine as low DPI (even my laptop is only around 150 DPI) but I still need to set that manually.
So no, GTK didn't keep the font rendering settings I care about, and is actively hostile in trying to ignore them even.
[1]: https://wiki.archlinux.org/title/GTK#Text_in_GTK_4_applicati...
That setting is there in GNOME Tweaks; I'm looking at it right now.
(I'm not trying to dismiss the concern, here, but AFAICT the setting isn't missing.)
Also Gnome Tweaks is an add-on, not part of the default install on any Linux distro I have tried. In KDE Plasma this is in the standard settings.
And if you use any desktop environment other than GNOME, their preferences certainly won't set the GTK4 hidden ini setting. Rather they expect programs to follow best interoperability practises, such as respecting the fontconfig settings. Most people won't limit themselves to only first-party apps from their DE of choice, so interoperability is king.