The definition of spline in the parent class (C++) is
UPROPERTY(VisibleAnywhere, BlueprintReadWrite, Category = "Orbit")
USplineComponent* Spline;
I don’t know what to make out of this deprecation warning.
EDIT: Also, the code effectively doesn’t execute. No spline points are created.
Only a guess: Is it private in c++ ?
protected
Similar to rubm123 - Check that your UPROPERTY is not a private: or protected: member.
There is however a way to expose a UPROPERTY that is private with a meta tag - but you will not be able to edit it in blueprint - it will be effectively ReadOnly and you wont be able to use EditAnywhere or BlueprintReadWrite meta tags.
At the moment, it just looks like none of my edits in the C++ source file is reflected in the UE editor. Really weird. I closed the UE editor, ran build in Rider, started the editor, and no matter how nonsenical my code (I made it that it still compiles), in the UE editor everything still works where really there are just null pointers.
I deleted the binaries folder but that doesn’t seem to be enough.
… and the problem seems to be that “clean” doesn’t do anything (yet still claims “success”). I have a “ERROR_UBTMissing” which seems to be a bug in UE5.0. I can’t find anything more about it, though.
And that was the solution to the root problem.
Very obscure issue.
Glad I could help. 
FWIW I just looked through the UE source and this change was apparently committed 25 days ago, it just hasn’t made it into the public release yet for some reason.
Hopefully Epic expedites that commit merge so it doesn’t cause too many other people trouble as well.