On 5 March, Gowling WLG published a briefing on this term and what it means. “Retained EU law” is a legal term introduced into UK law under the European Union (Withdrawal) Act 2018 (and also used in Isle of Man law too). It captures EU-derived rights and legislation the government intends to retain and preserve in UK law for legal continuity after Brexit. There is no specific list of retained EU law for lawyers to refer to – it is a matter of statutory interpretation.
https://gowlingwlg.com/en/insights-resources/articles/2019/retained-eu-law-a-practical-guide/