I have compiled a list of GTK2 programs (with some GTK3 ones that maintain some backwards compatibility). Feel free to let me know of programs that use GTK2 so that I can add them to the list!
I thought like that too, but then I realized that for a lot of things I would rather have a clicky-click GUI. I guess that the whole "FOSS GUI" incompatible toolkits disaster (GTK?, Qt?, FLTK?) gave everyone a PTSD.
Missing Terminal emulators. I like Terminator the most. Lilyterm is also fine, but much less featured.
Parcellite clipboard manager.
Old versions of GParted surprisingly fill the need. There might be some bugs or important additional features in the newer ones, I haven't checked.
After testing I prefer Viewnior as the image viewer. Mirage has an annoying "bug" where enlarging PNGs applies some filter and it can't be disabled (in a way that works, at least for me in Slack).
gcolor2 color picker (tiny and seemingly irrelevant but very useful for making website themes, unironically).
gtkman man page reader. Makes searching easier at least, than the terminal.
Old versions of hardinfo.
gftp is bugged but works well enough for me to use it regularly.
old versions of meld.
Zenmap 7.9.2 was still gtk2 funnily enough. Pretty recent.
xsane - seems they all use gtk2? Never used it though.