Fans are associated with reduction in SIDS, presumably related to improved room ventilation. But what about air purifiers, since COVID-19 these have been popular, is there any harm or benefit in using these air quality devices for newborns in the safe sleep environment?

Red Nose does not have any specific evidence based information on Air purifiers & SUDI (Sudden Unexpected Death in Infancy)

The information about the use of fans & any reduction in infant death is mostly related to babies who were placed on their tummy for sleep or when other cause of reduced air flow was evident.
Fan use may be an effective intervention for further decreasing SIDS risk in infants in adverse sleep environments & rooms where there is minimal circulation of air.

An oscillating fan positioned away from the cot can help during the hotter months. Fans circulate the air and also provide white noise for baby, which can help them sleep more soundly. Research has demonstrated that oscillating fans can help to reduce the risk of Sudden Infant Death Syndrome (SIDS) in some circumstances.

https://rednose.org.au/article/how-to-dress-baby-for-sleep

The 6 Safe Sleeping recommendations are accepted in producing an 85% reduction in SUDI rates.