The repository for WPF for .NET Core is linked right here at the top. You can explore it yourself. It only supports Windows (there's a lot of hardcoded Win32 stuff in it). Microsoft has said they won't spend resources to support cross-platform efforts on it. They also likely wouldn't stop community forks from trying (just don't expect MSFT employees to participate during day job time), but again you can explore the repository here and get a sense of how much work it would take (a lot).

.NET Core has always had platform-specific libraries that only work on a single platform or work somewhat differently on different platforms. The UI libraries (WPF and WinForms) are just the biggest examples now of platform-specific libraries on .NET Core.