In addition to the <Requires> tag (if you use it), plugins also have an option in addition to <Requires>. This is <RequiresFile> eg:
<RequiresFile>/var/log/plugins/unassigned.devices.plg</RequiresFile>
When this is used (in addition to Requires), if this file does not exist then the installation button and a reinstallation will not be allowed at all. MinVer and MaxVer take precedence over this (and if not met, the app won't even show up). If this condition isn't met, the app will still appear, but no option to install it will be given until the file exists (eg: until Unassigned Devices is installed)