It's been known for a while that lead actor Jodie Whitaker and showrunner Chris Chibnall were leaving Doctor Who and former showrunner Russell T. Davies was returning. There was a bit of a question mark over whether incumbent companions Mandip Gill and John Bishop would also be leaving or continuing into Davies' second era, but that now seems to have been resolved. Bishop posted a Twitter video in September confirming he is wrapped on the show, and Mandip Gill has followed it up in Doctor Who Magazine's 2022 Yearbook (published today) by confirming she is not returning. The BBC has so far not confirmed the news.
The news was fully expected: unlike the classic series, the rebooted Doctor Who has tried to arrange a "clean sweep" of introducing a new Doctor and new companions simultaneously whenever a new showrunner takes over. This happened in both 2010, when Steven Moffat took over with Matt Smith as the Eleventh Doctor and in 2018 when Chris Chibnall took over the show with Whitaker as the Thirteenth Doctor. In addition, John Bishop is one of Britain's most famous, successful and in-demand comedians and TV presenters, so was unlikely to take a long-term career break to do Doctor Who.
It does mean that Mandip Gill will manage the feat of fully matching the tenure of the Doctor she started with, previously only achieved by Billie Piper as Rose Tyler, who was companion for the full length of the Ninth Doctor's tenure (albeit that was only a single season) and a full year beyond, alongside the Tenth Doctor. Gill's character, Yasmin "Yaz" Khan, debuted in the Thirteenth Doctor's first full story, The Woman Who Fell to Earth, in 2018, and will depart in her final episode, a special set to air around October 2022 which also sees the Thirteenth Doctor's regeneration into the Fourteenth. This will make Yaz the longest-running Doctor Who companion of all time in terms of tenure (serving over four years), although only around the ninth-longest-running in terms of screen time, just beating Amy and Rose, but not quite matching Clara among modern companions, and some way behind the likes of Jamie and Sarah Jane Smith among classic companions.
This will allow Russell T. Davies to start with a completely blank slate for his first episode (already written), which will serve as the show's 60th anniversary special and is already slated to air in the week of 23 November 2023. Davies's return is also accompanied by the news that production (but not ownership) of Doctor Who has been outsourced to the independent production company Bad Wolf Productions in a deal which will hopefully allow for a larger budget and more editorial independence from the BBC. Production of Russell T. Davies's (second) first season is expected to begin next year.