prefix and ext seem to be return parameters, so need an r_ prefix.

Here you can just return false;
The check on totfiles is also independent of the bool is_multiview line above it, so by moving the if-statement one line up, things get even clearer:

{
  if (totfiles > 1) {
    BKE_scene_multiview_view_prefix_get(scene, name, prefix, &ext);
    if (prefix[0] == '\0') {
      return false;
    }
  }
  else {
    prefix[0] = '\0';
  }

  bool is_multiview = (seq->flag & SEQ_USE_VIEWS) != 0 && (scene->r.scemode & R_MULTIVIEW) != 0;
  return is_multiview;
}

(or remove the bool is_multiview variable altogether, also fine by me)

I'm a bit puzzled by the role of prefix and the rest of the logic here. It would seem that the returned value can be true even when totfiles == 1, and it can then also have prefix = "".

This is probably my lack of knowledge about the whole multiview system, though. Is it correct that you can have multi-view with an empty prefix?

The code seems to be semantically equivalent to the old code, though.

Maybe move this line to directly above the if (s_elem == NULL) { function, making their relation clearer.

Now there are two very similarly named functions, seq_render_movie_strip and seq_render_movie_strip_view. I think a comment is in order to explain what exactly this function is doing, i.e. how "view" in the name relates to "monoview" or "multiview".

This can be condensed even more into:

bool is_multiview_render = (seq->flag & SEQ_USE_VIEWS) != 0 &&
                           (context->scene->r.scemode & R_MULTIVIEW) != 0 &&
                           BLI_listbase_count_at_most(&seq->anims, totfiles + 1) == totfiles;

This probably should be its own function for clarity. It can then be placed next to seq_image_strip_is_multiview_render() so that it's clearer how these two functions differ, even though they basically try to express the same thing. I'm fine if you do that in a separate commit, though, as the way it is now is easier to review.

This is a functional change. Previously the NULL-check was only done when seq->views_format == R_IMF_VIEWS_STEREO_3D. Even though the new code may actually do the correct thing, separate functional from non-functional changes into separate patches. Or add a note to explain how this is a non-functional change after all ;-)