The authors looked at all systematic reviews (excluding methodological reviews) published in a few key journals as well as a random sample of Cochrane reviews to see how often the authors tried to search for unpublised data. The answer is not often enough (64% or 130/203). The article also describes the success rate in getting unpublished data when the attempt was made (89% or 116/130) and how often authors found evidence of publication bias when they did such an assessment (40% or 27/68). Although some people have argued that it is not that important to search for unpublised data, this is still a big concern. A closely related article is Searching for unpublished data for Cochrane reviews: cross sectional study.
Hedyeh Ziai, Rujun Zhang, An-Wen Chan, Nav Persaud. Search for unpublished data by systematic reviewers: an audit. BMJ Open (2017); 7(10). Available in html format.
You can find an earlier version of this page on my blog.