On May 1, Karim Asad Ahmad Khan, Special Adviser and Head of the United Nations Investigative Team to Promote Accountability for Crimes Committed by Da’esh/Islamic State in Iraq and the Levant, submitted a report to the UNSC. Part of it includes information about CW use by ISIL in Iraq:

A new investigation into the development and use of chemical and biological weapons by ISIL in Iraq has developed rapidly. Through the collection of a diverse range of evidence, the Team has confirmed the repeated deployment of chemical weapons by ISIL against civilian populations in Iraq between 2014 and 2016, as well as the testing of biological agents on prisoners.

I may be forgetting something, but I don’t recall seeing that last claim made anywhere by an official organization.

 There’s more:

31. The Team opened an investigation into the alleged development and use of chemical and biological weapons by ISIL. Initial work has been focused on lines of enquiry associated with the attack against Tazah Khurmatu, the ISIL takeover of the University of Mosul and its subsequent repurposing in support of a weapons development programme, and the attack against the Mishraq sulfur field and processing facility. Alleged attacks reportedly resulted in civilian deaths and injuries, including by asphyxiation and other chemical and biological weapons-related symptoms, affecting minority communities in particular. Evidence already secured indicates that ISIL tested biological and chemical agents and conducted experiments on prisoners as part of this programme, causing death. Weaponized vesicants, nerve agents and toxic industrial compounds are suspected to have been considered under the programme. 

32. To date, the Team has collected and analysed information and evidence emanating from battlefield records, detainee testimonies, victim accounts, satellite imagery, remote sensing techniques and video analysis. Aspects related to materials procurement and logistics, physical infrastructure, leadership and expertise are also being analysed. 

33. With this material, the Team has been able to identify repeated successful deployments of chemical weapons by ISIL against civilian populations between 2014 and 2016. Investigations into the successful development and use by ISIL of an indigenous chemical weapons capability may represent an unprecedented moment for accountability in modern conflict with respect to non-State actors.

One last item: “An initial case brief addressing the development and use of chemical and biological weapons by ISIL in Iraq will also be finalized in 2021.”