Requesting runs tagged with 'curves' through the API gives a 'no results' reply after ~2 minutes.
E.g. the following link (using the read-only API key): https://openml.org/api/v1/xml/run/list/limit/10000/offset/0/tag/curves?api_key=c1994bdb7ecb3c6f3c8f3b35f4b47f1f

However, runs do exist and can be retrieved through the web interface: https://www.openml.org/search?type=run&from=100&q=tags.tag%3Acurves

See also the information in openml-python#654.