International Organisation for Standardisation exists for one reason and one reason alone. It is to define and promote standards. On the one hand it does a wonderful job and on the other hand it has been set up to fail miserably.
Defining a standard is hard work and a political process. Once it has been defined, it is ratified, it has to be adopted. The politicians, that make the process extra hard have for reasons that are ... political, decided that ISO has to pay for itself and consequently it was told to raise funds by charging people for learning about standards.
The consequence is that people make do with other standards or substandard descriptions of the standard like this one for the ISO 8601. Substandard because it is not the standard.
Wikidata includes information about events, it knows about a start date and an end date. This is what the ISO 8601 is about: date, time, calculation of periods and obviously the presentation of all that. Obviously, it is easy enough to buy one copy of the text of the standard. But at the same time, there are many people, projects that are not as fortunate. They are anxious about implementing what is already defined in a standard, an ISO standard.
It is wrong headed politicians that ruin what is important. The politics of standards should be about the macro effects; the adoption of standards. It should not be about micro managing a system and breaking the system in the process.