Semantic Web technologies include for instance SPARQL, RDF, etc.
Are all questions about these technologies OK? (I hope yes, personally)
Or are these questions accepted only if they are used on open datasets? (which might be hard to tell)
In my opinion, they generally should be on topic here on OpenData.SX.
(Disclaimer: I currently write my PhD thesis on a Semantic Web topic, so I might be a bit biased.)
There could be edge cases that might be off-topic. When the question deals with a complex programming problem related to a Semantic Web technology, it might be a better fit for StackOverflow (see for example https://stackoverflow.com/questions/tagged/sparql with currently 1,439 questions).
On the other hand, questions about Linked Data (https://opendata.stackexchange.com/questions/tagged/linked-data) are a perfect fit for OpenData.SX.