I noticed a lot of backdrop constants moved over to
SharedXML\NineSliceLayouts.lua a while ago. Gethe has commits on it going back to 9.1.5.
These are applied using
NineSliceUtil.ApplyLayout().
BackdropTemplate is basically a NineSlice frame, so you can pass it directly to
NineSliceUtil.
If you insist on using XML, here are a few templates that might interest you.
Ultimately, it's
NineSlicePanelTemplate/
NineSliceCodeTemplate that handles applying the layout. You can inherit
NineSlicePanelTemplate directly and set the
layoutType key there if you aren't going to use the
backdropColor or
backdropColorAlpha keys.
If you're interested in how
BackdropTemplate converts the old backdrop tables into NineSlice layouts,
BackdropTemplateMixin:ApplyBackdrop() handles that passing
BackdropTemplateMixin:SetupPieceVisuals() as the optional setup function.