Why do the A220 cockpit seats have a cutout?
As we have seen in a previous question, the A220 is a re-branded Bombardier CSeries. 

The CSeries started flying in 2013. At the time Bombardier's other commercial jet was the CRJ, that started flying in 1999, 14 years prior.

CRJs do have a yoke:


(wikimedia.org)

If you look closely, you'll notice that the seats look identical, indicating that Bombardier probably decided to cut on costs by simply re-using something they had already (and minimizing the parts they would have to support).

Moreover, this answer mentions the need to have the straps as close to the pilot as possible, and this is achieved with the notch. A redesign of the seats has probably been deemed not worthy of the costs.