I think deleted scenes can be canon if the only reason they were cut was for timing. They don’t change the story dramatically but they might enhance certain aspects eg In Thor: the dark world, there is a deleted scene where Thor is speaking to his mother about Loki and whether or not she regrets teaching Loki magic. Frigga replies no as Thor and Odin “cast large shadows” and she wanted to help Loki “find some sun for himself”. This scene doesn’t alter any of the events storywise in either Thor movies but it does enhance why Loki is so driven to do what he does.
An example where deleted scenes perhaps shouldn’t be considered canon is X-Men: days of future past where Rogue was supposed to have a bigger role but her scenes were cut and thus we got the movie that we did. A “Rogue cut” of the movie was later released on DVD with the deleted Rogue scenes put back into the film which IMO actually made for the better movie but having her scenes in the film changed the story considerably (though did give an explanation for why the sentinels were as lethal as they were). It’s up to the audience to decide whether they want the original cut or the Rogue cut to be canon but the original cut did come first.
With OUAT, it’s difficult because you get some deleted scenes that should’ve been in show but were cut for timing eg Emma talking to Henry after Neal’s death. You get some scenes that are more or less useless or their absence doesn’t affect the story in any way eg Rumple taking something from rip van winkle. Then you get some deleted scene which would contradict something later on in the show but then don’t matter because they were deleted scenes eg Originally Rumple’s dad wasn’t going to be any more than some guy that tried to abandon Rumple before he was killed but this was later changed to Pan being Rumple’s dad.
IMO whether or not a deleted scene is canon depends on if it was only cut for timing reasons and if it enhances certain aspects of the story without changing the story dramatically.
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Keeper of Felix