This is a pretty big rumor, as at the time of htis writing, there is exactly one ‘outlet’ running a story about it, with absolutely no sources. The story comes from UK tabloid The Mirror, and claims “Doctor Who” star Jodie Whittaker has quit the series after only 3 seasons as the Time Lord.
According to The Mirror:

One insider said: “It’s all very hush-hush but it is known on set that Jodie is leaving and they are gearing up for a regeneration.
“Her departure is top secret but at some point over the coming months the arrival of the 14th Doctor will need to be filmed. It’s very exciting.”
Far as we can tell RIGHT NOW, this is not a factual claim.
We will let you know what we hear.