Did some more research on this issue. Since GTK 2's font rendering is done by Pango, Cairo, and Fontconfig I reinstalled those packages to make sure they are clean. Experimenting with ~/.font.conf shows that GTK 2 font rendering follows it for AA and hinting, but not subpixel hinting.
Then I compiled freetype 2.1.9 from the slackbuild with bytecode interpreter on, still no subpixel hinting in GTK apps.
Does anyone else have the same problem with GTK apps not using subpixel hinting?
The problem could be caused by Pango no longer directly accessing Freetype internals (as the freetype website says) and thus failing to utilize subpixel hinting. But I can't tell until I determine whether my problem is specific to my system or not.
Help a guy out and tell me whether I have a fsck'd installation please?