The GDPR is rather ambiguous about use cases like this. That's part of the problem. Different national regulators are going to have to decide what they consider reasonable and how much they want to enforce. Unless and until they issue authoritative guidance, everyone is basically in the dark on what this really means, and the penalties for misjudgement could be severe if the regulator strongly disagreed with your interpretation.

As for sharing data between organisations, one of the more unwise (IMHO) aspects of the GDPR is that when it comes to the right to erasure, the obligations to notify other parties to which the data has been supplied have guard conditions about reasonable efforts, but there are no such conditions at all on the immediate obligations to delete the data. If you're not covered by one of the acceptable cases (where the scope is, as noted above, not entirely clear yet) then you must be able to delete the data on demand, regardless of the practicality or cost of doing so.

It allows for simple logs. It does not allow for simple immutable logs, because your customers must always be able to force you to forget about them. There's nothing about sharing data between organisations: the GDPR requires — with certain very minor exceptions — that someone holding personally-identifying information delete it on request.

As an aside, this:

> This actually enables further innovations for instance you can log a cars behaviour without a user fearing you might sell this information on to their insurance company.

… isn't really true: the logger can still sell it, it just may not.

> That’s wrong. Data protection legislation expressly forbids

I'm sorry, I assume that English is not your first language. 'Can' means 'is able to'; 'may' means 'is allowed to.' The GDPR does not allow selling something, but it's still perfectly possible for someone to do so. People break the law all the time.

I raised the issue of can vs. may because it's germane to the issue of immutability. It is possible to build a system in which it is mathematically impossible (within arbitrarily-determined limits) to alter data; it is not possible to design a system in which it is possible to prevent someone from telling someone else something he knows. It's odd to me that the GDPR bans the former, desirable system.

GDPR has expectations that Data Controllers take reasonable steps to protect a data subject. Theres no expectation that you anticipate all mathematical possibilities...