I honestly don’t know if this is a Bug, or a Documentation problem (e.g. this big change was made, but never mentioned in release notes).
<https://xojo.com/issue/67088>
Edit:
- Per DesktopHTMLViewer can't use Native in 2021r3? - #3 by Anthony_G_Cyphers it looks like this is an intentional change
- If you want to see the “native” feature come back (but this time, using WebView2) there is a FR open: <https://xojo.com/issue/59961>