Map items at specific index

If I have an array of arrays, or a mapping, I'm looking for a version of map that will get me the items at specific index for the subarrays, or all the items of a key->item mapping.

For example, if I have an arr.yml file that looks like this:

key1: "item1"
key2: "item2"

or

- ['key1', "item1"]
- ['key2', "item2"]

I don't see any way to get just [ "item1", "item2" ].

Something like the following would be very useful:
{{ arr | index: 1 }} or {{ arr | map: '[1]' }}

I can't find any current way of doing this, except to use Jekyll support for push, i.e.:

{% assign tmp = "" | split: "," %}
{% for p in arr %}
    {% assign tmp = tmp | push: p[1] %}
{% endfor %}

The issue names no files or tests. Start by tracing the existing map behavior and the Jekyll push workaround, then compare the mapping and nested-array examples. Done means the requested filter behavior is defined for both forms and covered by tests.