As long as Silverlight is downloaded from an official download channel, such as the Microsoft website or through a Windows update, it should be perfectly safe. Due to the nature of the app, failing to keep it properly up to date could make it unsafe, so always install new updates.
Microsoft announced the end of the Silverlight application in 2012, ceasing development the following year. Patches and bug fixes will continue to be released as late as 2021, after which the application will be completely unsupported, and should no longer be used. The app is already unsupported in some browsers, including Edge.
On a Windows machine, the easiest way to tell if Silverlight is installed on your system is to simply type it into the search bar. If it appears as an app (not a web search result), then it is installed on your system. For internet browsers, check your installed plugins.
Google Chrome is not presently compatible with Microsoft Silverlight. In 2013, the Chrome 42 release disabled NPAPI by default, which prevented apps like Silverlight working. From Chrome 45 onward, the app is completely unsupported. With Microsoft discontinuing development on the framework, it is unlikely support for it will be re-introduced to Google Chrome.
The Microsoft Silverlight browser plugin is freely available for any supported platform. That being said, most up to date Internet browsers do not support the plugin, as it is now a deprecated framework that Microsoft ceased development on in 2013. It is still available to download for free from Microsoft's website.
Silverlight is as safe on a Mac system as it is on any other. With a platform like this, however, it is crucial that any new updates are installed, as they may close security holes that have been discovered. Most modern internet browsers no longer support this deprecated plugin, however.
The Netflix browser experience can work with either HTML5 or the Silverlight plugin. That being said, the list of operating system/Internet browser combinations which support Silverlight is diminishing over time, so while Netflix still supports the plugin, that is no guarantee that you'll be able to watch Netflix using it.
Silverlight does not come as part of Windows 10 itself, however you may find your Window 10 does have it installed. This is to do with Internet browsers. If you have a browser which supports the plugin (such as Internet Explorer 11), then the plugin may be included in your regular Windows updates.
Firstly, check if Silverlight is installed by searching for it in your installed applications. Next, find out if you are running an Internet browser which currently supports the plugin. If you are unsure, try finding a site which uses the platform, such as Netflix, and see if it will run.
As Microsoft announced the End of Life for Silverlight in 2013, and most Internet browsers have ceased supporting the plugin, you do no need this framework in order to have an enjoyable Internet browsing experience. Some cases may still require the plugin, but these would be exceptional circumstances, not the norm.