Hi Dazhi,
Thanks for reaching out, that is a good question. We expect contributions that are novel in the TRL community, being 1) entirely new papers, or 2) relevant work recently published in for example DB and NLP venues. As NeurIPS main conference papers are presented at the same conference just a few days before the workshop, these papers, if relevant to TRL and shortened to 4 pages (by NeurIPS policies), are still considered new papers and relevant to be discussed in the focused TRL workshop. Papers already presented at ML conferences (ICML, ICLR, etc) will have reached the community already over a longer time window hence lose novelty.
In my view, ICML, ICLR, AAAI are ML conferences, but CIKM and WSDM seem to serve a sufficiently different community and can therefore be acceptable.
Regards,
Madelon