There have been debates about whether or not the Defenders series of shows are canon within the MCU. When they aired, they made a lot of references to the events of the films, but they never actually crossed over into the official canon.
I've been hearing this for a long time, but the debate is over. Disney Plus has adopted Daredevil, Jessica Jones, Iron Fist, and The Punisher into its catalogue, and they are starting to appear in official MCU properties. The same versions of these characters are used to fight another common argument about canon. They are not the same actors who played them in the past.
Wilson Fisk said that this was the same version of Kingpin from Daredevil that showed up in Hawkeye. It's not just his head canon.
I'm not sure how you get more official than that, but they have an elaborately written character section for most of the MCU characters. If you go there, you'll get to see Kingpin and Matt Murdock's entire bio from their time on the show, as well as their new outings in Spider-Man: No Way Home and Hawkeye.
The characters are the same.
The former shows are part of the MCU.
The characters are not from a different universe. They keep their old series' storylines and events.
The end.
I am less sure about the later seasons of Agents of SHIELD. The ABC show that left for Disney Plus was more tied into the MCU in its early seasons, but as the show went on, it became free to do its own thing. The awakening of Inhumans across the entire planet was something that SHIELD never dealt with in the MCU.
It's possible that those characters could return to the MCU at some point, because SHIELD branched into time travel and a multiverse type situation. Everyone is dying to see Quake return in some part of the MCU. I would argue that the question of whether or not everything is canon in Agents of SHIELD is more complicated than the question of whether or not the Defenders is canon.
The Defenders are canon. Expect to see more of them in the future.
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The Herokiller series and The Earthborn trilogy are my sci-fi novels.