I find this behaviour really shocking. Academics should have standards around bias, transparency, evidence and ethics.
The review was commissioned by the NHS, so presumably entirely funded by tax payers, to review evidence to inform policy around some of the most vulnerable people (children) in this country. The letter as I've seen it expresses opinions but also makes unfounded and very serious allegations around the quality of research. Academic activists have been laying the grounds to discredit this report for months.
This is way beyond to rights to freedom of speech. I am hoping this is investigated and that there are consequences for signatories commensurate with the mischief intended to be caused here.