The update checker for Linux works incorrectly, it checks the Geany repository for a new version, not the repositories for the current distro. Most distros don't update versions of software like Geany on LTS versions for example. So it just feeds frustration for users of such
I'd say it depends on the user. There is maybe a minority of users who compile Geany themselves from release tarballs and for those the plugin might be useful.
Its really only useful for Windows, but maybe it could be made useful for Macos if it looked at the right repository because it can get new bugfix releases separate from Geany.
Isn't the plugin for Macos in the same way useful as for Windows as it checks geany.org for the latest version and this is where the Macos version is distributed. AFAIK Geany is not mainly distributed via some sort of repository like "brew" or so but via downloadable installers from our website and so I thought the update checker plugin would do the same job on Macos as on Windows.