Just a quick explanation of why the AN-2 Colt isn't such an easy target -
Low speed - It's low speed means two things, 1st that it can fly very, very low, making it hard for radars to spot and 2nd modern airborne radars have a feature called a "speed gate" which removes false returns caused by fast moving items like cars, trains etc and the aircraft can easily fly at just 90kt so the low speed of the AN-2 falls into this speed gate making it "invisible" to a radar unless the radar is specifically "ungated" which adds so much extra clutter for the operator to fight through to find the actual aircraft (they'd usually try to follow roads/railway lines etc).
IR & Noise - It has a very low IR signature due to the air cooled radial engine and a very low noise signature, making it very hard for MANPAD troops to find and lock-on a modern IR guided SAM onto it which is looking for jet engine temps of around 850'c+ (older SAMs) or 200'c+ for newer IIR guided SAMs which detect the heat of a leading edge as cuts through air creating friction.
So not only is hard to see and engage on a radar but its also hard to hear, or lock-on with an IR missile which makes it sound great but why isn't it being used. Well, all those advantages would disappear against a "lower tech" enemy like Ukraine because they generally use a lot of optically aimed rapid firing cannons (23mm to 35mm usually) alongside all those modern missiles and these old cannons would easily knock out this old aircraft.
Nothing a twin PM1910 Maxim can't deal with
