libgtkmm-2.4-doc 1:2.24.5-4ubuntu2 (amd64 binary) in ubuntu focal

 Gtkmm is a C++ interface for the popular GUI library GTK+, with API version
 2.4. Gtkmm provides a convenient interface for C++ programmers to create
 graphical user interfaces with GTK+'s flexible OO framework. Highlights
 include type safe callbacks, widgets extensible using inheritance and over 180
 classes that can be freely combined to quickly create complex user interfaces.
 .
 This package is deprecated. Please see the libgtkmm-3.0-doc package for
 the documentation of a newer version, which wraps GTK+ 3 instead.
 .
 This package contains FAQ, and reference documentation.