The Gimp Toolkit (GTK) predates the GNOME project, thus it became GTK+ awhile when it was adopted by the new GNOME desktop (Gimp+GNOME Toolkit) before being returned to just being called GTK in time (GNOME dominate the development of it now).
GTK1 (1998-04-13) & GTK2 (2002-03-11) are now deprecated
GTK3 (2011-02-10) is currently in maintenance mode
GTK4 (2020-12-16) is the current and development version.
However programs/apps that use the specific toolkit/libraries won't all be converted/ported all at the same time, especially those maintained or developed by volunteers, thus apps will use a mixture of various toolkits (ie. those that have been updated, those that haven't).
Libraries/toolkits get removed; usually that'll be Ubuntu & Debian at the same time, eg. I'll provide a post to the removal of Qt4 ( Removing Qt 4 from Ubuntu before the 20.04 release - Foundations - Ubuntu Community Hub ) which in time I expect will happen with GTK2, the question is mostly when.
Yes you could build a distro or flavor using only one subset of the repository, for Ubuntu (or flavors) that involves either being careful with what is included on the seed, or just blacklisting what you don't want included (so builds will break if you include, or force to be included something you've blacklisted). That however would limit what packages could be included (ie. far fewer options for users to choose from).