It depends what aspects of the iOS plugins you want to test: native or hybrid (JS)?
I.e. do you want to debug the Javascript running in the hybrid WebView - that of your own application and/or the Javascript layer of the plugins?
Or do you want to debug the native layer of the plugins (Android => Java, iOS => Objective-C/Swift)?
Bear in mind that any sort of deployment to an iOS device requires membership of Apple's iOS Developer program in order to be able to sign the app with valid credentials.
With regard to Phonegap Build, while it's possible to build an iOS app with Phonegap Build and receive the compiled .ipa (iOS equivalent to .apk), it's not easy to then directly connect an iOS device to a Windows 10 PC and connect a remote debugger to attach to the iOS Webview that your Cordova app is running in. I read that jsconsole offers one such free solution, but I've not tried it (there's a blog post about it, though).
One solution I have used is not free: Telerik Appbuilder is a paid product. It's Windows classic IDE allows you to build, deploy and debug the hybrid Webview of Cordova apps on an iOS device from a Window 10 PC using their cloud build service.
However, if you want to debug the native elements of iOS plugins, at this point your are going to need XCode (Apple's official IDE for developing iOS/OSX apps) and therefore the OSX operating system. But before you go out and buy a $1000 Macbook Pro, I would suggest one alternative that I use to develop both native and hybrid iOS apps on a Windows 10 PC: although it's not officially sanctioned by Apple, it's possible to run OSX inside a virtual machine using virtualisation software such as VMWare Workstation or Virtual Box - also known as a "Hackintosh". This allows you to use a Windows 10 host machine to run the OSX operating system and gain access to all of its features. By connecting an iOS device to a Windows 10 PC running an OSX, you can connect XCode to the device and use it's interactive debugger to debug the native elements of Cordova plugins. Also, you can use the Safari Browser's dev tools to connect to the iOS Webview and debug the JS layer.
I have used this method to develop and deploy both native and hybrid iOS apps to the App Store using a Windows 10 PC. However, bear in mind that a virtualised OS runs slower than a native one: I would recommend running the OSX image off an SSD with a high-performance PC: i7/32+Gb RAM/high-end GPU.