Collaborator servers lack ipv6 support
No IPv6 support for any of the collaborator infrastructure:
burpcollaborator1.portswigger.net has address 126.96.36.199
burpcollaborator2.portswigger.net has address 188.8.131.52
Knowing an ipv6 source address for originating traffic is far more useful than a legacy ipv4 address, as it is less likely to be nat'd behind a shared address and less likely to be discovered via traditional scanning methods.
Thanks for your feedback. We’ve made a note of your request in our development backlog.