I'm quite sure the last time I installed Debian, a year or two ago, I didn't choose a DE so it installed no DE at all.

Are you people talking about installing from a GNOME-based live image or something? I think most people use the netinstall image, which uses no DE and, I'm about 90% sure, has no graphical environment selected by default.

This is correct, but lacking context: If you don't select the desktop task, no desktop is installed; if you select the desktop task, but no specific version of it, it installs GNOME.

There are also KDE, XFCE, LXDE, and maybe others, to also install; but the user has to select those instead of the default desktop task.

Perhaps you used the minimal image, I remember both having a DE when installing debian and not having a DE when I installed it again later.

If you choose the desktop environment task then by default it gives you GNOME.