Silly me. I see the "delete forever" option now. Need more coffee, I think (or maybe less, not sure!).
The webhooks are just doing their job via Listeners and actions to dispatch when those events are received. Something else (possibly some other extension) maybe at fault, or some weird issue with core (beta 15).
I disabled a fair number of extensions, but my non-admin user can still see those soft-deleted discussions. I haven't looked, but I assume the admin user can see them always. Double checked and my non-admin user is not in the admin group.