GTK 4.0

> Does Debian consider that GTK 2 is in the archive solely in order to support other Debian-packaged applications, or is also there so Debian's users can run their own programs which might have been written against GTK 2?

A bit of both. If you're developing your own programs written against GTK 2, then when it eventually gets dropped from the archive, you're well-placed to pick up responsibility from its Debian maintainers and compile your own copy of GTK 2 alongside your programs, if that's what you need. We provide you with full source code, both for the library itself and the packaging, and even if its upstream developer no longer does; so if it has bugs that are a showstopper for your particular use, then you have the option of fixing them, even if the fix is an incompatible change and you end up maintaining a fork to go with your programs.

We can't maintain superseded libraries or other runtime environments forever with a finite number of developers, and I don't think we should pretend that we can. It seems better to have a clear signal that we are no longer taking responsibility for a particular library, giving people who depend on it a chance to either pick up responsibility for having their own version, or move away from it.

There's nothing particularly special about GTK here: as a distribution, we've done this in the past for GTK older than 2, Qt older than 5, Python 2 older than 2.7, Python 3 older than 3.9, Perl older than 5.32, Ruby older than 2.7, GStreamer older than 1.0, and many more.
(Version numbers taken from testing/unstable and the planned Debian 11 stable release; adjust downwards as appropriate for the Debian 10 stable release.)

In practice, an old .deb from an old Debian release will often install and work acceptably on a newer release (at your own risk, no support implied), so continuing to use an unsupported library doesn't *necessarily* mean compiling your own - but if you want bugs fixed, or if you want to rebuild against newer dependency ABIs, at some point that becomes your responsibility rather than the distribution's. Anything else is just not sustainable.