In what capacity?
The built-in APIs for handling images (GDI+) added WebP support in 1809. This is mentioned in the the documentation for the drawing library in C#/.NET, which is a wrapper around GDI+: https://learn.microsoft.com/en-us/dotnet/api/system.drawing.imaging.imageformat.webp?view=net-8.0
I haven’t tried it in the Photos app recently, and don’t have a Windows system handy right now to try it out, but I know it works for sure in Paint, which uses GDI+ for image encoding/decoding (and thus supports all formats that Windows itself supports).
Of course, apps that use their own encoding/decoding code might have fewer features and support fewer formats than the native Windows code.
Edit: Apparently it works fine in the Photos app if you just change the extension? Which means Microsoft is doing something weird. The code to decode WebP must be there.
