>>16810102
Knowledge of feathered dinosaurs existed since the 1850's, shortly after "dinosaur" was even coined as a term and still before the scientific process as we know it today.
We more recently discovered that more dinosaurs had feathers than we thought (though, to be clear, most dinosaurs probably didn't have feathers) because those groups had more subtle feathers that didn't fossilize as well.
Basically, we knew theropod dinosaurs were the ancestors of birds since we first started studying dinosaurs. We just underestimated how long ago feathers developed.