3.1 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 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 (like wpa_supplicant_gui, keepassxc, picard) even though in logs they seem to load the plugin, dont theme accordingly and show the default white color. 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?

I Can confirm this if i run

QT_DEBUG_PLUGINS=1 QT_QPA_PLATFORMTHEME=gtk3 wpa_gui 2>&1 | grep -i gtk3

and logs show

QFactoryLoader::QFactoryLoader() looking at "/nix/store/64qmp7fap3740hxd2bsnpy10s9ndy5qx-qtbase-5.15.18-bin/lib/qt-5.15.18/plugins/platformthemes/libqgtk3.so"
Found metadata in lib /nix/store/.../libqgtk3.so, metadata=
            "gtk3"
    "className": "QGtk3ThemePlugin",
Got keys from plugin meta data ("gtk3")
loaded library "/nix/store/.../libqgtk3.so"

• Note: The old qt5-styleplugins do work just need to set the variabled to gtk3 instead of gtk2

Side by side comparison

• qbittorrent (qt6 app)

Its using qt6, i can confirm
nix why-depends nixpkgs#qbittorrent nixpkgs#qt6.qtbase
/nix/store/i8r2n73xzlxk7sfchjlkdxajs8jbqbv0-qbittorrent-5.2.1
└───/nix/store/wzvm2pvf98a71v0scgfhmi01lqcxahfr-qtbase-6.11.0

• picard (qt5 app)

Its using qt5, i can confirm
nix why-depends nixpkgs#picard nixpkgs#qt5.qtbase
/nix/store/wzqlnryvnjl95a93fiwwivlbz3ypixk7-picard-2.13.3
└───/nix/store/64qmp7fap3740hxd2bsnpy10s9ndy5qx-qtbase-5.15.18-bin

I can confirm the same behaviour for the other aforementioned apps.