I suppose it's interesting to find out whether and what companies are A/B testing, not sure you can learn a great deal though as each site will have its own objectives, KPIs, design and user segments. Rotating banners or versions of the same is really just a way to have more content and more dynamic design but its not really A/B testing.

By the way - if you are thinking of A/B testing, my article has some serious considerations to make before you do:

https://kolemcrae.com/testing-sucks-right/

(2009)

That said, it's fascinating how archaic websites from just 9 years back look today..