There is no need to sign any additional NDA statements when dealing with Support. Please see below 3 relevant policies that can be referred to if and when customers ask for specific NDA's before engaging with support.
Employment contracts: All employees (including Support) have signed employment contracts with confidentiality clauses in them. Confidentiality clauses are the same as NDAs for this purpose.
Contract with the customer: The customer contract will say that any information passed from Qlik to the customer (and vice-versa) if it is confidential data, will be treated by the other party as confidential.
Qlik Handling of 3rd Party Data Policy: This policy reminds employees that customer data is confidential and summarizes the procedures to keep it confidential and safe (e.g. data retention rules).
Where are the data retention rules and methods to encrypt them ? Is there a document showing how long the log data we provide to Qlik support is retained and in what format etc. and when does the data completely gets removed from your system.
Support follows our company-wide privacy policy, which you can review here: Legal Agreements. You can find this referenced in the Support Policies (point 7.8) in Product Terms.