If possible, do not call UIDropDownMenu_Initialize directly but instead set some attributes. Below is an example for such changes in playerDropdown.lua.

local function Startup()
    Dropdown = CreateFrame('Frame', 'BagnonPlayerDropdown', UIParent, 'UIDropDownMenuTemplate')
    Dropdown:SetID(1)
    Dropdown.displayMode = 'MENU'
    Dropdown.initialize = UpdateDropdown
    return Dropdown
end

Calling UIDropDownMenu_Initialize can cause taint?

Dam, taints everywhere, everywhere!
Actually, it's about time I put Bagnon running on Sushi instead of classy, which comes with non blizzard dropdowns (there could still be taint after using more than 8 buttons).

Thanks for the tip!

Done