I started working on this bug but fortunately discovered fairly quickly that upstream is on the way of moving to GTK+ 3:
https://sourceforge.net/p/linsmith/discussion/389360/thread/90b515fb/#01e5/f302 You might want to check the status, it would be a pity if this package has to be removed for buster. Even if the port is not completely finished, please encourage him to publish the work either as a VCS branch or as a patch so that we can help get it done.
