QGtk3Style
This is a platform theme built into qt5-base starting with version 5.7.0 [2] and qt6-base. It can be used to style Qt5 and Qt6 applications according to current GTK3 style. It can be enabled by setting the following environment variable: QT_QPA_PLATFORMTHEME=gtk3. 

I am on nixos and while if i set the same envionment variable qt6 apps like pcmanfm-qt, qbittorrent correctly use the same colors as my gtk3 theme, but it seems qt5 apps even though in logs they seem to load the plugin, dont theme accordingly and show the default white color. Its an old thread so I just want to know is the plugin abandoned? Meaning its no use trying to wait for it to update and be fixed or should i just switch to qtstyleplugins for qt5 apps?

As for the ldd output, for some reason I'm getting a significantly different ouptut from when I tried with 5.8. I might have run it with a different application last time or maybe I ran it on a debug build. This is what I got running it just now with 5.9:

linux-vdso.so.1 =>  (0x00007fffe6d46000)
libQt5Widgets.so.5 => /home/user/Qt/Qt5.9.0/5.9/gcc_64/lib/libQt5Widgets.so.5 (0x00007f3117c9f000)
libQt5Gui.so.5 => /home/user/Qt/Qt5.9.0/5.9/gcc_64/lib/libQt5Gui.so.5 (0x00007f31174f2000)
libQt5Network.so.5 => /home/user/Qt/Qt5.9.0/5.9/gcc_64/lib/libQt5Network.so.5 (0x00007f3117160000)
libQt5Core.so.5 => /home/user/Qt/Qt5.9.0/5.9/gcc_64/lib/libQt5Core.so.5 (0x00007f3116a28000)
libstdc++.so.6 => /usr/lib/x86_64-linux-gnu/libstdc++.so.6 (0x00007f3116681000)
libgcc_s.so.1 => /lib/x86_64-linux-gnu/libgcc_s.so.1 (0x00007f311646a000)
libc.so.6 => /lib/x86_64-linux-gnu/libc.so.6 (0x00007f31160a1000)
libpthread.so.0 => /lib/x86_64-linux-gnu/libpthread.so.0 (0x00007f3115e84000)
libGL.so.1 => /var/lib/VBoxGuestAdditions/lib/libGL.so.1 (0x00007f3115c4b000)
libm.so.6 => /lib/x86_64-linux-gnu/libm.so.6 (0x00007f3115942000)
libz.so.1 => /lib/x86_64-linux-gnu/libz.so.1 (0x00007f3115728000)
libicui18n.so.56 => /home/user/Qt/Qt5.9.0/5.9/gcc_64/lib/libicui18n.so.56 (0x00007f311528e000)
libicuuc.so.56 => /home/user/Qt/Qt5.9.0/5.9/gcc_64/lib/libicuuc.so.56 (0x00007f3114ed6000)
libicudata.so.56 => /home/user/Qt/Qt5.9.0/5.9/gcc_64/lib/libicudata.so.56 (0x00007f31134f3000)
librt.so.1 => /lib/x86_64-linux-gnu/librt.so.1 (0x00007f31132ea000)
libdl.so.2 => /lib/x86_64-linux-gnu/libdl.so.2 (0x00007f31130e6000)
libgthread-2.0.so.0 => /usr/lib/x86_64-linux-gnu/libgthread-2.0.so.0 (0x00007f3112ee4000)
libglib-2.0.so.0 => /lib/x86_64-linux-gnu/libglib-2.0.so.0 (0x00007f3112bd2000)
/lib64/ld-linux-x86-64.so.2 (0x000055a3ba2f4000)
VBoxOGLcrutil.so => /usr/lib/x86_64-linux-gnu/VBoxOGLcrutil.so (0x00007f31129ef000)
libXcomposite.so.1 => /usr/lib/x86_64-linux-gnu/libXcomposite.so.1 (0x00007f31127eb000)
libXdamage.so.1 => /usr/lib/x86_64-linux-gnu/libXdamage.so.1 (0x00007f31125e8000)
libXfixes.so.3 => /usr/lib/x86_64-linux-gnu/libXfixes.so.3 (0x00007f31123e2000)
libXext.so.6 => /usr/lib/x86_64-linux-gnu/libXext.so.6 (0x00007f31121cf000)
libpcre.so.3 => /lib/x86_64-linux-gnu/libpcre.so.3 (0x00007f3111f5f000)
libX11.so.6 => /usr/lib/x86_64-linux-gnu/libX11.so.6 (0x00007f3111c24000)
libxcb.so.1 => /usr/lib/x86_64-linux-gnu/libxcb.so.1 (0x00007f3111a02000)
libXau.so.6 => /usr/lib/x86_64-linux-gnu/libXau.so.6 (0x00007f31117fd000)
libXdmcp.so.6 => /usr/lib/x86_64-linux-gnu/libXdmcp.so.6 (0x00007f31115f7000)

qgtk3 plugin

I think you are right.
Also i have this file in 5.7
"C:\Qt\5.7\Src\qtbase\src\plugins\platformthemes\gtk3\qgtk3theme.cpp"
So I guess something else is wrong.

The ldd output of this thread is still valid ?
https://forum.qt.io/topic/75398/qt-5-8-0-qapplication-invalid-style-override-passed-ignoring-it/9

Also seems that Creator itself suffer from it
https://bugreports.qt.io/browse/QTBUG-53844

So not sure if its fixed and just bugged on some distros or not fixed
at all.

I don't think they'd include it if the plugin had been abandoned. Plus, I don't think they could abandon GTK theme emulation entirely since it's essential to make Qt applications look native on Linux.

Maybe its just old info :)