If I wanted to render a legacy WPF app on a view within a new ASP.NET Core MVC application what would be the modern approach for this? Is this sort of thing still possible? Previously the obvious solution would have been Silverlight I guess.
Asp.Net core does not directly support silverlight or WPF. If you were able to get a silverlight app to be served up from an ASP.Netcore view I suspect you'd have to do it in some manually way because the tooling isn't going to help.
There really isn't any good way to render a legacy WPF app on a view using asp.net Core MVC. Even if you did port the WPF app to silverlight and managed to get the silverlight app to play nice with an ASP.Net Core view, you will have put in all that work to port to a technology (silverlight) that isn't accessible via many mobile devices and is likely to be phased out over time.
It may be worth thinking about what would be required to port the WPF app to Asp.Net Core MVC if you want the WPF app functionality available via a browser. That porting work would be more challenging than porting to silverlight but in the end you'd end up with an app that was on technology that has a much brighter future.