I am unable to completely remove the windows-slaves-plugin.  When I remove it either in the UI or within the linux environment, on restart, the plugin is back and re-enabled. These are the steps I used to reproduce the issue: (attached screenshots of each of the steps below)

1. The alert in my Jenkins instance showed that this plugin is depreciated.
2. In the plugin manager UI, I clicked the disable plugin toggle.
3. I get the warning that I'm uninstalling the agents plugin, which I select yes.
4. I then click the restart once no jobs are running button in the UI
5. After restart I see the plugin is back and enabled.

I also repeated the steps above on the command line by doing the following:

1. Disable the plugin in the UI
2. Stop jenkins in linux

sudo systemctl stop jenkins.service

1. I removed the plugins.  

   rm -rf window-slaves.jpi
   rm -rf windows-slaves.jpi-disabled
   rm -rf windows-slaves (the directory)
   

1. Restart Jenkins
2. windows-slaves plugin is back and running again