system
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human
[Doctor Who] How come the Doctor thought he was the only Time Lord?

Pre-50th Anniversary, the Doctor was always swanning on about how he was the last Time Lord. So did he forget that his granddaughter was a quick flight away?
gpt
There are a few hints that he went looking for Susan in his Ninth incarnation and couldn't find her. A short story also suggests that he might have tried again in his Current incarnation.

We don't know what has become of her. She may have been summoned or taken back to Gallifrey or something.

The post-timewar version of the universe is separated from the pre-timewar version due to the time lock on the war itself, and there are times. places and people - including other Time Lords - who are simply in timelines that the Doctor cannot access any more.

Now that Gallifrey is back, it is possible that its re-emergence may have triggered yet another version of the universe to supercede the  last, and make further time-lines inaccessible - whilst possible opening some that had previously been locked.