"data type" is a very unclear term, and it's not obvious how it would be different from a "value type". "Property type" is much more descriptive, since it actually defines the type of a property.
I believe the fact that we have a concept called "data type" always was and still is an issue. A minor one, but still. Even I, as one of the senior developers, always struggle when explaining the difference between "data type" and "data value type".
I agree it's of not much value to go around and change one term to the other. But we should make sure we are preferring more descriptive vocabulary when introducing new code. In code documentation and variable names we are doing this and slowly moving towards "property data type" whenever it makes sense.
/me also agrees in part
We will always have the term "DataType" unless we want to make breaking changes, for example, it is in our API output, and a bunch of other places
Had we done this when we first thought of the name change it may have made sense, but it would be quite a shake up now for code using wikibase and also peoples knowledge of wikibase.
We could try to come up with some migration plan but I honestly don't think it is worth it.
I'll check back soon and potentially close this ticket
Personally, I don't think it makes sense to touch the string datatype in any machine-readable output. (In other words: I don't think this is worth a breaking change.) But I think it makes sense to prefer $propertyType as a variable name in code, and "property data type" in documentation. Especially the later. What can you loose if you do this?