The taboo might not be common to all the Ds.We even had a D inside the Marines (Jaguar D Saul) and neither his family name nor his initial were hidden. I just think that some of the Marines are not very bright, and correct me if I'm wrong but wasn't the Monkey D in Dragon not mentionned beforehand?He was known as Kakumei Dragon(Revolutionary Dragon). It was until Garp specifically told Luffy that his father's name in Monkey D Dragon that everyone made the connection between Garp,Dragon and Luffy.They simply lacked that information about Dragon's name, and who can blame either of them for distancing themselves from each other? Garp was still a Marine after all, and Dragon is openly opposing the WG, so it wouldn't look good for Garp that his son became a revolutionary. And the RA are known for discarding their past, and family as seen with Gaburu.
More importantly it's not like there's only one line of Monkeys, what if Garp had a brother or something? And in rl some family last names can be common, but they aren't directly related to each other. The D and more importantly the fact that there is a clan of D suggests they're all related one way or another. I agree that given the context of the Ds being God's natural enemy, they might be related to the Ancient Kingdom. As for Water which again is his mother's name which may or may not be a D, it can be taboo for different reasons, that might not be related to the D. In fact he was about to explain why his family hid that name, but he was interrupted by Baby 5 and Buffalo so we didn't get the full story.
There's a chance it might become relevant in the future.
---------- Post added at 02:00 AM ---------- Previous post was at 01:32 AM ----------
When it comes to
naming taboo, it's something that existed in China, but also Japan, and these days it has more to do with the Emperor himself. I could see something like that applying here, and yes it could just be Law who has a taboo name. In which case it's not related to the D clan and the Ancient Kingdom, but his own family on his mother's side most likely.