This news comes from Patrick K. Mills, who is the senior quest designer for CD Projekt Red. He discussed the length of the main story on a Twitch Livestream. A clip was posted over on Resetera.
During the Livestream, Patrick K. Mills noted that the primary campaign for Cyberpunk 2077 would be shorted due to complaints from The Witcher 3 players. Many players never reached the end of the game due too the overall length of how long the story was, and they did not want that to happen in Cyberpunk 2077, so they shortened the main story.
Due to Cyberpunk 2077 being an open-world game, the amount of content available far exceeds what is available through the primary story campaign. Even if the story is shortened, there is still quite a bit side content to play and explore both during and after you complete the main story. I am sure their decision to shorten the main story will not affect the overall player experience.
Cyberpunk 2077 is set to release this November 18th, 2020.