Besides the out of universe reason that his identity was hidden as a plot device, is there any in universe reason people don't know Naruto is the 4th Hokage's son?
After all, Minato was married to Uzumaki Kushina. She must have been pregnant for a while before her and Minato's death. Being the wife of the 4th Hokage, she must have been, at least to some degree, a celebrity figure. Everybody would have known she was pregnant. She must have been sporting a good baby bump for at least a couple months before her death. Shortly after the time of her death, there's suddenly a newborn baby named Uzumaki Naruto wandering around (that happens to have a striking resemblance to the 4th, which becomes more and more obvious as he ages), and nobody made the connection they were related?
Suddenly orphaned, Naruto was left to grow up knowing nothing of his parents, receiving only his mother's last name because the Third Hokage thought that it was best that nobody knew that he was related to the Fourth Hokage.
I know that the Third hid his relation to the 4th, but simply using the 4th's wife's last name doesn't seem like it would be very effective. Was that really all it took to fool just about EVERYBODY? Is there some other explaination to Naruto's anonymity?
there were still some Uzumaki in the land of Fire probably nomadic, but I don't think that's accurate. There are only scattered survivors and Kushina was selected to be the new jinchūriki and came to the village BEFORE the Uzumaki clan was destroyed. While I'm not saying there CAN'T be other Uzumaki clan members wandering around, there is no evidence for that.