The Elder Scrolls VI was announced during Bethesda's E3 press conference back in 2018. It was then confirmed to be in pre-production, and thanks to the LinkedIn page of a Bethesda employee working with Talent Acquisition, we now know it still is:
"Bethesda Game Studios sets the benchmark for open-world gaming and is currently in full production on Starfield, its first new universe in 25 years, and pre-production on the highly-anticipated The Elder Scrolls VI."
A game in this franchise does require an enormous amount of pre-production as it simply has so much content that also needs to be tied together. A reasonable guess is that large chunks of the team that are now finishing Starfield will start the full production of The Elder Scrolls VI once the former has been released on November 11 this year.