They abandoned it and decided to add at the last minute variant walkers, which I thought was stupid because they decided to do it too late (season 11). Variant walkers are walkers but more advanced as they can climb and run. It's clear they are milking the shit out of this show with multiple spin offs. Spin offs galore.

They abandoned it and decided to add at the last minute variant walkers, which I thought was stupid because they decided to do it too late (season 11).