This has been the same claim with every iPhone and every time it's been unfounded.
Well, not totally. A lot of iPhones have availability launch weekend and then backlogs grow as Apple tries to distribute inventory world wide. Last year, for example, the iPhone 7+ was not easily available until late November. Before that time it was only randomly available, usually not in the color or size or carrier you needed.
Of course that is very different from these rumors saying no availability until Nov/Dec. I don't buy that at all.