Dont think one has been closed. They have just not (yet) been needed. Staffing them is a different concern mind.
NEC confirms full reopening, with NHS Nightingale being decommissioned - Conference News
In a letter to its clients, the Birmingham venue noted that the field hospital has been in hibernation since May and did not receive a single patient
www.conference-news.co.uk
London's Nightingale hospital on standby amid rising Covid-19 patient numbers | ITV News
Concerns have been raised around the already-stretched health service’s ability to staff Nightingale facilities. | ITV News London
www.itv.com