When we talk about distortion with wide angles lenses, it’s all relative; “low distortion” can mean “quote noticeable warping” with some subjects. Very few wide angle lenses are free from distortion; the most common is barrel distortion (outward bowing), but “wave” or “moustache” distortion is becoming much more common with advanced designs. Remember, optical design is all about compromises, and it’s hard to banish distortion without resulting in some other reduction in performance; lens designers would eliminate distortion if it were easy!
Shown below is the distortion graph for the Zeiss ZF/ZE 21mm f/2.8 Distagon. On a smaller sensor (Nikon DX or Canon EF-S), distortion will appear as barrel distortion. On full frame, distortion will be visible as wave-type distortion. Compare the distortion graph to that of the Zeiss ZF/ZE 85/1.4 Planar and ZF 100/2 Makro-Planar, then see the examples that follow.
The examples below show the wave-type distortion which can be of concern for architectural photography, whether at distance or closer range.