GTK-2 has reached EOL, most applications should be now using GTK-3+.
Fcitx, IBus, SCIM and UIM, in relation to GTK(+) and Qt, provide primarily not applications using GTK(+) and Qt, but plugins internally loaded by GTK(+) and Qt which provide input methods support in all applications using GTK(+) and Qt. Support for plugins for GTK+ 2 in Fcitx, IBus, SCIM and UIM should be package.use.masked at the same time as package.masking GTK+ 2 itself, not earlier. However possibly existing GUI applications (usually for configuration, like fcitx(5)-config-*, fcitx(5)-configtool, ibus-setup, scim-setup) can unconditionally use newest/best version of GTK(+) / Qt.