There is no singular canon, only what is convenient for the episode. You're right about Treehouse episodes being "less canon" than most others, but there very few things that are permanent in the show that haven't been changed or couldn't be changed in a future episode. So it's a sliding scale of "canonism"
We can't count tree house of horrors episodes.
What do you mean, they have numbers in the episode title
It’s understood that the events of Halloween episodes aren’t canon. They’re like Futurama What If episodes.
It's a joke about counting stuff with numbers in the episode name
There is no singular canon, only what is convenient for the episode. You're right about Treehouse episodes being "less canon" than most others, but there very few things that are permanent in the show that haven't been changed or couldn't be changed in a future episode. So it's a sliding scale of "canonism"