We recently deleted about 150k leads from our Marketo database as they are unsubscribed or very inactive. Does Marketo keep these contacts unsubscribed if we added them back in through a list upload? Is there a way to upload a list and not have it update any lead fields or add any new leads?
Those are two separate questions.
1) If you delete them out of Marketo (and not Salesforce) then sync them back over from Salesforce, the unsubscribe will remain. Otherwise, Marketo will no longer 'know' if they were unsubscribed because you deleted them (and their lead history).
2) You can upload a list and not update fields by managing your preferences in the admin section. You can identify which lead sources you want to update leads and which lead sources you don't.
as a complement to Kristen's points :
Hope this helps,