Scite Reference Check
A commercial subscription product. There is a limited free level at the time of this writing (2/2021) "Launched in November, Scite Reference Check scans new article PDFs for references to retracted papers, and posts both the citing and retracted papers on Twitter. It also flags when new studies cite older ones that have issued corrections, errata, withdrawals, or expressions of concern. An extension of scite.ai, a platform that monitors whether research findings are supported or contradicted by subsequent studies, Scite Reference Check keeps track of retracted papers by trawling through Crossref, PubMed, and the Retraction Watch Database."