I did send an email to DAL (see 9.8 of their privacy policy) based on the The EU general data protection regulation 2016/679 (GDPR), still have the emails;

Dear Sir/Madam,

I am a customer of yours, and with reference to articles 12 and 15 of the General Data Protection Regulation (AVG/GDPR), I am making this request for access to personal data.
I request that you provide the following information to me:

-Please provide me with the categories of personal data/information you (and your partners such as SkyMiles and KLM) have about me in your files and databases.

-Please provide me with a copy of, or access to, my personal data that you have or are processing (as described on your website as):

9. YOUR RIGHTS

This section 9 applies only to individuals in the European Union and other countries which grant the rights described here. These rights will apply only in certain circumstances.

9.1. Requesting a copy of your information

To the extent you are legally entitled, you can request confirmation of whether or not we process your personal information, and details of the information that we hold about you and how we use it.

You also have a right to access your personal information and to be provided with a copy.

I appeal for my request based on Article 12 and 15 first paragraph of the General Regulation data protection (AVG).
I would like to receive an answer from you to the above request within a month & a copy of all personal data that is processed. Based on the general data protection regulation you may not charge me for this.

Kind regards/Vriendelijk gegroet,
xxxxxxxxxxx
xxxxxxxx
xxxxxxxx
Kingdom of the Netherlands

Skymiles# 1234567890

and added an edited copy of my passport to that email as further proof. Around 29 days later i got an email;

Dear Grouchy,

Using the identity information you provided to us, below is a secure link to the data file in support of your Subject Access Request submitted to us on xx/xx/20xx.
https://xxxxxxxxxxxxxxxxxx

It is protected by a password which I will share with you separately.
The link will accessible for 30 days.

Our Privacy Policy provides you with detail on the data we collect, why we collect it and how it is used.
If you have any question on the content of the file, please let us know.

Sincerely,
xxxxxx

Posted some info and some screenshots last year in this post:

https://www.flyertalk.com/forum/30635864-post8.html