This week we announced Partial Leads in the Lead Manager CRM. This will allow importing leads and manually creating leads that only require one of the following: first name, last name, email, phone or address.
From the API point of view for a lead to be created you must provide at least one of:
source field is still required.
And for integrations receiving leads from our Outgoing API keep in mind that you could receive a lead being created missing those fields.