Throughout the episode the Doctor and Bill argue about how real she is. Even after she is revealed to be a glass avatar, Bill insists that since she is the sum of the character’s memories, she isn’t a duplicate. Both Bill and Clara’s fates have already been complicated by their unnatural resurrections, but the memory extraction provides at least a theoretical possibility that this time their resurrection is eternal. It’s as close to a concept of the afterlife as Doctor Who is ever likely to get – the heavenly equivalent of Missy’s hell in Death in Heaven, and the real world equivalent of River Song’s virtual resurrection in the library.
Although the Doctor misses out on the chance of one last hug with Clara, he embraces Bill and Nardole despite remaining unconvinced that they can be described as the real deal. A touch of guilt remains for the Doctor, given that Nardole’s appearance here suggests that he did indeed eventually die along with the children he stayed to protect on the Mondasian ship.
A common criticism of Steven Moffat’s tenure has been his alleged propensity for undoing the finality of death. But such a charge has not stood up to scrutiny until now. Danny Pink, Clara and Bill all died. Danny refused the chance to be resurrected, Clara knew she must return to Trapp Street after her stay of execution, and Bill will have almost certainly chosen to return to her mortal life on Earth despite the offer made to her by the Pilot.
After death a person can only live on through the memory of others. Testimony, whilst questionably bypassing consent allows the memories of the Doctor’s friends to live on unmediated. The Doctor probably doesn’t accept this as being alive, because in his estimation we are not memories but stories.