As a follow-up to myself, it now appears to be IWF policy that members must implement the blocklist https://www.iwf.org.uk/members/member-policies/url-list/tes

As a follow-up to myself, it now appears to be IWF policy that members must implement the blocklist https://www.iwf.org.uk/members/member-policies/url-list/testing-policy https://www.iwf.org.uk/members/member-policies/url-list/test... which presumably explains why some previous 'observer' members are no longer members. Again, though, IWF membership is not mandatory for any organisation.