[Fizzlemizz]

First decision. Do you want to make seperate xml frames for classic and non-classic

If yes then:
Classic: (same as always)

<Frame name="TestFrame" parent="UIParent">
    <Size x="300" y="300"/>
    <Backdrop bgFile="Interface\Tooltips\UI-Tooltip-Background" edgeFile="Interface\Tooltips\UI-Tooltip-Border" tile="true">
        <BackgroundInsets>
            <AbsInset left="5" right="5" top="5" bottom="5"/>
        </BackgroundInsets>
        <TileSize>
            <AbsValue val="16"/>
        </TileSize>
        <EdgeSize>
            <AbsValue val="16"/>
        </EdgeSize>
    </Backdrop>    
    <Anchors>
        <Anchor point="CENTER"/>
    </Anchors>
    <Scripts>
        <OnLoad inherit="prepend">
            self:SetBackdropColor(0.5, 0.3, 0.3)
            self:SetBackdropBorderColor(0.5, 0.3, 0.3, 0.25)
            print("Loaded!");
        </OnLoad>
    </Scripts>
</Frame>

Others, Mostly the same xml frame but inherits the BackdropTemplate and all backdrop information is set in lua (OnLoad)

<Frame name="TestFrame" parent="UIParent" inherits="BackdropTemplate">
    <Size x="300" y="300"/>
    <Anchors>
        <Anchor point="CENTER"/>
    </Anchors>
    <Scripts>
        <OnLoad inherit="prepend">
            self:SetBackdrop( {
                bgFile = "Interface\\Tooltips\\UI-Tooltip-Background",
                edgeFile = "Interface\\Tooltips\\UI-Tooltip-Border",
                tile = true,
                tileEdge = true,
                tileSize = 16,
                edgeSize = 16,
                insets = { left = 5, right = 5, top = 5, bottom = 5 }
            })
            self:SetBackdropColor(0.5, 0.3, 0.3)
            self:SetBackdropBorderColor(0.5, 0.3, 0.3, 0.25)
            print("Loaded!");
        </OnLoad>
    </Scripts>
</Frame>

Or, do you want one frame for both Classic and the rest: Mostly the same xml frame but detecting if backdrop mixin (template) is needed and all backdrop information is set in lua (OnLoad)

<Frame name="TestFrame" parent="UIParent">
    <Size x="300" y="300"/>
    <Anchors>
        <Anchor point="CENTER"/>
    </Anchors>
    <Scripts>
        <OnLoad inherit="prepend">
            if BackdropTemplateMixin then -- if this is NOT a Classic type frame, add the backdrop mixin
                Mixin(self, BackdropTemplateMixin)
            end
            self:SetBackdrop( {
                bgFile = "Interface\\Tooltips\\UI-Tooltip-Background",
                edgeFile = "Interface\\Tooltips\\UI-Tooltip-Border",
                tile = true,
                tileEdge = true,
                tileSize = 16,
                edgeSize = 16,
                insets = { left = 5, right = 5, top = 5, bottom = 5 }
            })
            self:SetBackdropColor(0.5, 0.3, 0.3)
            self:SetBackdropBorderColor(0.5, 0.3, 0.3, 0.25)
            print("Loaded!");
        </OnLoad>
    </Scripts>
</Frame>

Done this way to show separation of xml and lua. With the Classic and non-Classic you could use <KeyValues></KeyValues> for both but not with the one frame for multiple versions layout.

Definitions like BACKDROP_TOOLTIP_16_16_5555 and LEGENDARY_ORANGE_COLOR don't exist in Classic and may not BCC either so probably easiest not to use them and just define backdrop, colours, alpha manually in both.