The supplied Xfce4-hardware-monitor-plugin package for Debian buster has a dependency on libsensors4. That package does not exist in buster. There is a libsenors5 package.
There is no package for xfce4-hardware-monitor-plugin in Debian so I'm unsure where you got it from. I'm removing myself from CC: then.
I'm sorry. I downloaded the package from https://goodies.xfce.org/projects/panel-plugins/xfce4-hardware-monitor-plugin. That page linked to a readme at https://git.xfce.org/panel-plugins/xfce4-hardware-monitor-plugin/tree/README.md?h=omegaphil/pkg which said to report problems here. If there is somewhere else that I should be reporting the problem then I will be glad to do so.
Sorry Mike, its been about a year since I stopped maintaining this plugin.
This project has been archived. Closing bugs.