What exactly is the sitecore_list_index used for? Can it be deleted or can its crawling strategy be set to manual on CDs without affecting any other part of Sitecore?
sitecore_list_index contains the list of contacts and segmented lists that are used by the List Manager, the Email Experience Manager and the Experience Profile.
More information you can find here:
On this link https://doc.sitecore.net/sitecore_experience_platform/setting_up__maintaining/xdb/configuring_servers/configure_a_content_delivery_server you can see that you need to disable List Manager index on a content delivery server.