Test and add a backdrop frame in the OnLoad script:
This adds an extra frame for the backdrop rather than just inheriting the template.
Code:
<Frame>
<Scripts>
<OnLoad>
if BackdropTemplateMixin then
self.background = CreateFrame("Frame", nil, self, "BackdropTemplate")
self.background:SetAllPoints()
self.background:SetBackdrop({ ... })
self.background:SetFrameLevel(self:GetFrameLevel())
end
</OnLoad>
</Scripts>
</Frame>
Blizzard are not concerned with making addons compatible between Classic and Retail. Addon authors have to decide how far they are willing to go if that's what they want to do.