[Phanx]

Filter options for the default UI do not affect oUF.

However, on further investigation, oUF itself applies a filter if you don't supply one, and if you set certain properties on the Debuffs element:

local customFilter = function(icons, unit, icon, name, rank, texture, count, dtype, duration, timeLeft, caster)
	if((icons.onlyShowPlayer and icon.isPlayer) or (not icons.onlyShowPlayer and name)) then
		return true
	end
end

If you want to use this default filter instead of supplying your own, then I'd suggest that rather than removing the filter, you replace it with a dummy one that lets everything through:

local showall = function() return true end

local f = CreateFrame("Frame")
f:RegisterEvent("MODIFIER_STATE_CHANGED")
f:SetScript("OnEvent", function(f, event, key, state)
	if key ~= "LSHIFT" and key ~= "RSHIFT" then
		return
	end
	local a, b
	if state == 1 then
		a, b = "CustomFilter", "__CustomFilter"
	else
		a, b = "__CustomFilter", "CustomFilter"
	end
	for i = 1, #oUF.objects do
		local object = oUF.objects[i]
		local buffs = object.Auras or object.Buffs
		if buffs and buffs[a] then
			buffs[b] = buffs[a]
			buffs[a] = showall
			buffs:ForceUpdate()
		end
		local debuffs = object.Debuffs
		if debuffs and debuffs[a] then
			debuffs[b] = debuffs[a]
			debuffs[a] = showall
			debuffs:ForceUpdate()
		end
	end
end)