What is an "all-weather" aircraft?
“All-weather” is a little bit of a hold-over from the WWII days where severe weather was much more of a threat to the aircraft. Between less reliable engines, delicate and finicky instruments like artificial horizons, and limited anti-ice technology, flying in adverse conditions was much harder then. Becoming completely disoriented in low visibility was almost a death sentence.

Special planes were made with the sole purpose of fighting at night, and certain designs were better than others for all-weather operation.

Nowadays, world class military aircraft are all pretty much all-weather because of the 70 years of avionics, propulsion and airframe improvement.