This one is pretty straight forward and the title almost says it all.
In The End of Time part 2 when the 10th Doctor was dying and Ood Sigma came to see him. Sigma said “This song is ending, but the story never ends”……
Did he know the Doctor was fully immortal?
Of course this refers to the most hated story twist in Doctor Who. – Yes even more than some people hated Jodie Whittaker being the 12th Doctor. The idea that The Doctor was actually this truly immortal child found by the civilization that would eventually become the Time Lords. And they (The Doctor) has been regenerating thousands and thousands of times (millions?). And that They (The Doctor) where what inspired the Time Lords to develop their own regeneration process.