Pretty sure there is a Mode-S Beast built into the blue Flight Feeders, which do have filtering: Mode-S Beast:System Design - Beast Wiki
Maybe your old generation Flightfeeder does not have GPS and may would even be compatible with a filter but it still doesn’t need one.
On the units with integrated GPS antenna the filter would disrupt the GPS signal coming from the antenna.
Anyway when you have excellent coverage up to very high distances it is normally a sign that you don’t need any extra filtering.