RetractionBot
This is a Wikipedia bot which flags citations to references which have been retracted, as reported by Crossref. If a source is flagged as retracted through Crossref, any citations to that source should be flagged with an appropriate template for review within 24 hours.
It currently runs only on the English Wikipedia - at https://en.wikipedia.org/wiki/Special:Contributions/RetractionBot.
The idea for this bot came from discussions at WikiCite 2018, in addition to on-wiki discussions such as this one.