Like in topic title, is the SimpleLazyObject part of public API? It appears in some release notes, and its there on the Source code for the django.utils.functional but otherwise it gets no mention by the docs. 
I’d have to say it isn’t. (Simply being in the source code wouldn’t qualify - otherwise, everything would need to be considered stable unless explicitly identified.)
From the docs at API stability | Django documentation | Django
Stable APIs¶
In general, everything covered in the documentation – with the exception of anything in the internals area is considered stable.
From this, it’s reasonable to conclude that if it’s not explicitly mentioned in the docs then you shouldn’t consider it “stable”.
On the other hand, a rather cursory inspection of that code on github gives me the impression that the code has been stable for at least 10 years. I’d guess it would be a pretty safe bet to remain so for the reasonable future.