The Doctor Who season 11 finale was the first episode of Jodie Whittaker's Doctor Who era to make mention of a story featuring the Ninth Doctor, Christopher Eccleston. This bears mentioning as the season had made multiple references to stories starring the other "New Who" Doctors, seemingly leaving Nine out in the cold. This seemed doubly odd as Eccleston's Ninth Doctor and Whittaker's Thirteenth Doctor have a lot in common as characters.
When Doctor Who was revamped in 2005, Eccleston faced a daunting ask in reimagining the Doctor for a new generation. While the revival was dependent upon the work of hundreds of television professionals, Eccleston's performance would be the glue that held the whole production together and set the standard by which everything else would be judged. Thankfully, Eccleston met the challenge and his darker yet occasionally whimsical take on the Doctor secured the new seasons' place in the cultural zeitgeist as a science-fiction phenomena and a return to form for the classic family program. Whittaker has faced a similar challenge as the first female Doctor in the show's long history, and while her take on the Doctor is more light-hearted than Eccleston's world-weary warrior, she sports a similar edge when her good humor is broken.
The Doctor Who season 11 finale - "The Battle of Ranskoor Av Kolos" - found the Doctor and her companions working to thwart the villainous schemes of galactic tyrant Tzim-Sha (aka Tim Shaw) after answering nine distress calls emanating from the titular planet. In order to disrupt the signal that Tzim-Sha would use to destroy the Earth, the Doctor hit upon a scheme that would take advantage of the reality-rewriting powers of a race called The Ux and the telepathic circuits that powered the TARDIS. When the Doctor's companion, Yazmin, asked if this would work, the Doctor confessed that she wasn't sure, but noted that she once pulled a similar stunt with the telepathic circuits and "turned a Slitheen back into an egg." This was a reference to the Ninth Doctor story "Boomtown," where Eccleston's Doctor did indeed save the day by causing the villain to revert to an embryonic state, ensuring that he could see her properly raised to become a productive, non-murderous member of her society.
Granting that the Ninth Doctor-era doesn't offer as many opportunities for referencing due to its brevity, as Eccleston only played the Doctor for one season, it still seems odd that more attention has not been paid to it in the new season. Doctor Who season 11 has seen multiple references to the Tenth and Eleventh Doctors' adventures. Indeed, "The Battle of Ranskoor Av Kolos" sees the Doctor describing the events of the Tenth Doctor story, Journey's End, in which the Doctor used the TARDIS as a tow-truck to pull the Earth back to its proper place after being taken halfway across the universe by Doctor Who's Daleks.
Presumably this lack of attention to the Eccleston-era is the result of there being so little material to draw upon and not an intentional slight. Regardless of the reasons, it is nice to see the Ninth Doctor-era finally being referenced in this fashion. Hopefully, Doctor Who season 12 will see more nods to the Doctor who started the Who resurgence.