I’ve thought the same. I can see why they’ve added it though - far easier to pop off a rubber coolant hose and clean/replace a filter than strip the entire dashboard out to replace a clogged heater matrix.
But where is the debris coming from? Unless it’s a consequence of the split heat source setup on models with an electric heater - if the ICE never starts (short, gentle trips with no high heater demand) then the coolant in the engine and radiator will tend to sit around and maybe allows for some oxidation or something to take place that eventually gets flushed through to the matrix when the valve does get opened.
I wonder if the diesel Outlander has the filter?
2015 Mitsubishi Outlander PHEV GX4H (petrol/electric)
2008 Volkswagen Caravelle (diesel)
(recently departed - 2004 Subaru Outback 3.0Rn (LPG), 1991 Mitsubishi Pajero 2.5 LWB (WVO))