I haven't yet submitted any model-released photos to shutterstock yet, but have quite a few in my backlog that I think might work.

These are not "people photos", and have things like:
- Silhouettes
- Their backs to you (but not silhouetted)
- Blurred faces beyond recognition because of depth of field
- Body parts - like a hand reaching for something (no tattoos)

Are these acceptable without model releases?

You never know. Depends who is reviewing your work . I usually go the extra mile even if is not needed, just to be sure.Keep in mind that model release are not only for reviewers . Its a way to  keep your port safe for any infringement claims especially now. So if you have them use them

https://www.shutterstock.com/blog/protecting-your-content-learn-about-model-releases

Quote from: catavic on February 10, 2017, 03:53
You never know. Depends who is reviewing your work . I usually go the extra mile even if is not needed, just to be sure.Keep in mind that model release are not only for reviewers . Its a way to  keep your port safe for any infringement claims especially now. So if you have them use them

Yeah quite a few of these specific shots were while I was in Asia, so most of the time they were quick shots of people in action, or sometimes they'd even pose/allow the photo, but I never tried to get any releases since language barrier was massively difficult as-is...

If in doubt submit as editorial, from your description it sounds like editorial stuff anyway.