When you delete a record, and load new records, do the new records get assigned a deleted record's former lead ID? Or will that lead ID always be associated with the deleted record?
The deleted records will not inherit their old Id if uploaded again. It will assign a new ID.
There is a discussion related to this. Please refer to the below:
When you delete a record an upload the record again, new record gets created and New Marketo ID gets associated to it also make sure when you delete record from Marketo only and not from Salesforce the leads get sync back to Marketo from Salesforce and this may create duplicate leads.