Single Word Requests – Word for First Name and Middle Name/s, But Not Last Name

The best I can think of is to say that their names are disyllabic duonyms, making them disyllabic-duonymic.

Such an answer though is in the realm of fun word games with obscure terms, rather than anything of much use in real English use. Don't say anyone is disyllabic-duonymic, except perhaps as a joke, aimed at a rather nerdy audience.

euphemism - the substitution of an agreeable or inoffensive expression for one that may offend or suggest something unpleasant; also : the expression so substituted.

It's more commonly used in contexts where the "offensive" term is so-called because some (or many) other people consider it rude/smutty or sacrilegious. But effectively the word being avoided is taboo, whether that's because of general perception or the edict of an all-powerful ruler.