Even before spread syntax was being used in Azure Maps it was a ton of work to get it to work in the old WebView and then the performance wasn't great. The old WebView has very limited support of WebGL and the Azure Maps Web SDK makes heavy use of WebGL. I simply would not recommend using the old WebView. If WebView2 isn't an option (a bit surprised if it isn't), you could also use CefSharp which is a wrapper of chromium. I used the old webview in a windows app ages ago before WebView2 was available, and quickly switched to CefSharp as there simply too many issues with the old WebView. I've since switched to WebView2 and never looked back. I have a simple sample of Azure Maps in a UWP via WebView2 here: https://github.com/rbrundritt/SimpleUwpAzureMaps
I'm also working on a Maui wrapper that leverages WebView2 to expose a .NET API for the Azure Maps Web SDK.
What is the reason why you have to use the old WebView?