Good to see many philosophers are signatories.  From the letter:

We are also concerned about the suppression of proper academic analysis and discussion of the social phenomenon of transgenderism, and its multiple causes and effects. Members of our group have experienced campus protests, calls for dismissal in the press, harassment, foiled plots to bring about dismissal, no-platforming, and attempts to censor academic research and publications. Such attacks are out of line with the ordinary reception of critical ideas in the academy, where it is normally accepted that disagreement is reasonable and even productive.

Many of our universities have close links with trans advocacy organisations who provide “training” of academics and management, and who, it is reasonable to suppose, influence university policy through these links. Definitions used by these organisations of what counts as “transphobic” can be dangerously all-encompassing and go well beyond what a reasonable law would describe. They would not withstand academic analysis, and yet their effect is to curtail academic freedom and facilitate the censoring of academic work. We also worry about the effect of such definitions on the success rates of journal submissions and research grant applications from governmental bodies such as the AHRC and ESRC.