One wonders what window would do to modern air defences?
Bugger all - and that’s been true since the 1970s.
MTI, Doppler MTI*plus many other forms of signal processing will identify and remove such clutter - including bird formations, heavy cloud, Met balloons etc.
Same processing used on airborne radars to remove ground clutter, motorway traffic etc.
* And with a digital 3D radar they can be applied in thousands of discrete height layered squares to compensate for differing wind speeds and directions etc.