A big footgun with strncpy is that the output string may not be null terminated.
There’s languages where you can be quite confident your string will never need null termination… but C is not one of them.
char buf[12];
sprintf(buf, "%s%s", this, that); // or
strcat(buf, ...) // or
strncpy(buf, ...) // and so on..There are certainly ways in which the c library could've been better (eg making strncpy handle the case where the source string is longer than n) but ultimately it will always need to operate under the assumption that the people using it are both competent and acting in good faith.
Fixed size strings don’t save bytes on storage tho, when the bank reserves 20 bytes for first name and you’re called Jon that’s 17 bytes doing fuckall.
What they do is make the entire record fixed size and give every field a fixed relative position so it’s very easy to access items, move record around, reuse allocations (or use static allocation), … cycles is what they save.
I have seen plenty of fixed strings in the 8 to 20 byte range, not much, but often enough for a passable identifier. The memory management overhead for a simple dynamically allocated string is probably larger than that even on a 32 bit system.