Do Debian and Ubuntu even bother rebuilding packages for old releases?
No, the dependency only matter for users of older releases that would like to build a newer Geany by themselves. And Debian was just an example, sometimes we check what even more slow-paced distro do, like RHEL or so. Anyway, I don't think this should be a reason to go one direction or another myself, as staying with libsoup2 is causing issues on current and future distros, and again I won't add support for selecting the one to use myself.