Every frame used to have a backdrop if it was needed or not. Now they are optional (at least in retail, I don't think change has been backported to the classics yet but...?).
In retail, your frames that require backdrops can inherit the the new BackdropTemplate
Code:
<Frame name="somename" inherits="BackdropTemplate">
...
</Frame>
Code:
local f = CreateFrame("Frame", "somename", UIParent, "BackdropTemplate")
and if you want to add a backdrop to a frame that has already been created but hasn't interited the template then you can add the backdrop mixin (code) before setting the backdrop
Code:
if not frame.SetBackdrop then
Mixin(frame, BackdropTemplateMixin)
end
frame:SetBackdrop(...)