I can still use and develop GTK2 software in Debian out-of-the-box, but not Qt3 or even Qt4.

Yes, because GTK2 is so difficult to upgrade to new versions, whereas Qt4 to Qt5 usually only involved replacing a few identifiers (cannot speak to Qt6 since I don't do Qt anymore). That's why distros need to drag GTK2 along even though we are already at GTK4.

	▲	gen2brain 2 hours ago \| parent \| next [-]
		I work on the IUP fork, where I have and support the Qt driver, which works with both Qt5 and Qt6, and I only need a couple of checks to make it work on both. I often check if I broke the Qt5 build; nope, all good there, hard to break. For GTK3 and GTK4, those must be separate drivers; there's no way you can handle that with a couple of checks. Everything is different. That was possible to some extent and works with GTK2/GTK3 (single driver), but there are so many checks and workarounds there. Qt is so nice to work with.
	▲	kvemkon 4 hours ago \| parent \| prev [-]
		> so difficult to upgrade to new versions Perhaps, because GTK developers insist, that GTK2, GTK3, GTK4 are different toolkits. > Qt4 to Qt5 usually only involved ... I'd start with Qt3 to Qt4.