Hi,

For any model you can find the archived records using the filter and setting the value as Active False, you can see that here in this video: https://www.youtube.com/watch?v=pHkAZu9BMPQ

For the CRM there is a default filter for this, see the Archived Filter in the below image,

Thanks

If you are using Odoo 13, there is a filter in the list view of the leads. Apply the filter and you can see all Archived leads.