The Witcher 3 had great sidequests. They were self-contained short stories that made the world feel richer. In an interview with Gamingbolt CD Projekt RED level designer Miles Tost implies that Cyberpunk 2077's sidequests will be less self-contained, and might actually affect the critical path.
“Sidequests are great opportunities for us to tell stories of our cyberpunk world. After all, despite not being a part of the main questline, they are still connected to the world and can elaborate on certain parts of it, like the characters,” says Tost.
“So it’s all interwoven. And yes, that also means that our sidequests could affect the main story.”
Earlier this year quest designer Patrick Mills told us that to expect sidequests to be "something similar to The Witcher 3."
"Particularly in terms of playtime and in terms of quest complexity. Specifically, I'd even look more at the expansions than The Witcher 3 base game because that's really where the quest design philosophy that we're using now came from—it was developed later in the expansions."