Flexible meta-regression is an important facet of biomedical research. These methods employ basis expansions to model the relationship between exposure and outcome. Examples include splines, fractional polynomial, and local regression models. The extent to which flexible meta-regression has been appropriately employed and reported in the past has not been studied. This study aims to provide an overview of flexible meta-regression for researchers as well as summarize the practical use and reporting of these methods in the epidemiological literature. A systematic literature search of EMBASE, MEDLINE, and the Cochrane Library identified systematic reviews published between 2011 and 2021 which employed flexible meta-regression methods. Data on study characteristics were extracted and validated by two independent researchers. Given the lack of guidance on reporting standards for flexible meta-regression, we proposed a novel 9-item checklist. This was used to assess reporting quality in included studies. A total of $N=346$ eligible studies were identified and $N=86$ were randomly sampled for full data extraction and analysis. The number of reviews using flexible regression methods increased over time. Spline models were the most frequently employed class of methods. There were no apparent trends in the use of the three methods over country of affiliation, number of authors, or clinical context. In over two thirds of reviews sampled, methods were found to have not been reported consistently. Further work is needed to improve the reporting of these methods to ensure research transparency and reproducibility.