I would go with something similar to this:
{{ order.customAttributes | where: "key", "Delivery-Date" | map: "value" | join: "" }}
Why? To avoid unnecessary for loop. This is also a much more simpler to write as oneliner.
Idea is first find correct attribute by key using liquid where filter (https://shopify.dev/api/liquid/filters/array-filters#where) and pick the value. Don’t find any other liquid method/filter to return value than map it to new array and then join the found value(s) as string