Heres some further proof that Marvel: The End was cannon:
In the official bio's from the Annihilation series, when you look up Thanos, under Threat Level it says:
"Universal/One time reality ender"
Heres the scans, look under Threat Level:
Here it is outlined in red, so you don't miss it:
Now,
Thanos was a Universal Threat when he held the IG, and even the cosmic cube, but was he a reality ender at this point? ❌
The only time Thanos has truly ended all of reality was when he possessed THOTI. ✅
I'm not saying whether or not Thanos destroyed a Universe or a Multiverse, I'm just providing additional proof that "The End" arch was in fact cannonical.