Title says all. I'm merely curious if the events of SCP: Containment Breach are considered canon. If not, why?
Containment Breach is a fangame; therefore, not officially endorsed or associated with the site. So people are allowed to include it in their headcanons or not as they please.
Also, not a lot on this site is "canon", per se. Aside from the basic tenants (the O5 council, mobile task forces, Safe/Euclid/Keter (hell, some people don't even headcanon Thaumiel stuff to exist); just those very basic stuff), anything on the website can be considered canon or not by the individual user.
Reason: Nothing is canon. If it makes sense for you, it will be canon for you. But it might not be canon for others. Vice versa. We all have our visions of what the Foundation-verse is like.
If not, why?
As an analogy: if someone makes an awesome Star Wars fan video about a Sith called Darth Dunkel, Disney's not suddenly going to include in Star Wars Monopoly, no matter how awesome a lot of people think the video is.
If one does exist, I'd start here. Otherwise feel free to make your own, there is literally an entire cannon dedicated to cyborg cat girls and a wizard going on space adventures, so I'm sure no one would mind if you made one specific to SCP: Containment Breach
You're confusing - and granted, the terminology is confusing - the uncountable concept of canon (i.e. something is canon or not, part of the core mythology of the site) with the concept of a canon, a series of tales and/or SCPs all intricately inter-related in terms of core setting, narrative, and/or metaplot.
The OP is asking about the former, you're referring to the latter.
I'm sure no one would mind if you made one specific to SCP: Containment Breach
Well, caveat. You need more than one author and more than one mainsite page to make a site canon series/collaboration.
There's that, yes.