For a casual viewer like me — I haven’t seen any of the new episodes — I’m struggling to find what this is referring to.
What happened with Clara and the doctors, and Capaldi, and what was this thing that nullified 50 years of the show?
The nullified thing is known as the “timeless child” storyline. The Doctor is retconned to not be a Gallifreyan at all and also the source of everyone’s regenerative powers (or something, I honestly forget). It was a totally unnecessary retcon but IMO much worse it was incredibly self indulgent. Most of the audience cares very little about the Doctor’s backstory, they just want enough setup to enjoy the adventures.