In the early-mid 2000s, a lot of animation studios were hired to do experimental hybrid animation. A lot of them have this feel where because of advancements in video compositing like Adobe After Effects, the 2D characters have this smooth shadow. These 2D animated characters would be prepped and composited into a 3D sequence, and it contrasts well since the 3D isn't trying to blend in, unlike in tv with Fairly Odd Parents, Spongebob or Futurama
From the top of my head I thought of
>The 2003-2005 CN City/Adjacent bumpers
https://www.youtube.com/watch?v=Q6rBeODM2nc
>Gorillaz, specifically Phase 2 era
https://www.youtube.com/watch?v=HyHNuVaZJ-k
https://www.youtube.com/watch?v=cLnkQAeMbIM
https://www.youtube.com/watch?v=uAOR6ib95kQ
>Guitar Hero/Rockband Adjacent Opening Cinematics or Adverts
https://www.youtube.com/watch?v=rbe5mpI16KA
https://www.youtube.com/watch?v=zQtHkBh2bzY
https://www.youtube.com/watch?v=he8gQpNVrfg
Just asking around for anything I'm missing, I'm an animator trying to capture a similar feel by the way