We have now published our first research report, which examines whether the information that people provided on the Census was coded and processed accurately. The report of this investigation ‘Collecting and processing Census 2021 data on gender identity’ concludes that 94% of respondents to the Census answered the question and there is no evidence to suggest that the published results were not coded or processed accurately.
https://www.ons.gov.uk/news/statementsandletters/updateonresearchintogenderidentitydataincensus2021