Unable to remove item at non-existing index.

This is definitely an issue.
But patch #2 works only if you need to remove 1 element.
removeItem() function reassign deltas and for the next elements it won’t work.

The solution should be similar to the jsonapi module from Core:

1. Collect deltas to remove
2. Remove them in the desc order.

I tested with the multiple field reference to the paragraphs.
I have 3 paragraphs with 2 unpublished:

$items = [
  0 => 'paragraph-0-unpublished',
  1 => 'paragraph-1',
  2 => 'paragraph-2-unpublished',
];

Unpublished paragraphs are not shown in the $elements after viewElements() and should be removed from the $items

$elements = [
  1 => 'paragraph-1',
];

Created MR and added a patch.