The Guardian has always been a newspaper with politics most closely aligned with the Lib Dems. In the past, this meant that they were left of Labour. Labour has shifted waaay to the left[1][2], past the Lib Dems and past the Guardian, so the Guardian no longer finds themselves in political alignment with Labour. I don't think the Guardian have changed their political stance substantially.

[1] https://www.politicalcompass.org/images/uk2015.png [2] https://www.politicalcompass.org/charts/uk2017