They're attributive nouns.
Here, release is an attributive noun. It modifies the head noun directly following it:
Together they form the nominal release date, meaning "the date of [something]'s release". Add an article to that, and we get a complete noun phrase:
[ The release date ]noun phrase had yet to be determined.
Your example, though, is written in headlinese, a style in which articles and other words are often omitted. In this case, have been is also omitted (before unveiled).
It also uses a slightly more complex structure than the above: it coordinates release date with price to form a larger nominal, release date and price. This nominal is modified by another attributive noun, iPhone:
the iPhone [ [ release date ] and price ] have been unveiled
This could be considered a reduced version of two full noun phrases:
the iPhone [ release date ] ] and [ the iPhone price ] have been unveiled
I've bolded the attributive modifiers: iPhone, release, and iPhone again. The omitted words here are stricken through. The noun phrases mean in turn "the date of the iPhone's release" and "the price of the iPhone".
They're not adjectives.
Nouns can be used as attributive modifiers, just like adjectives. It's a basic function that nouns have, and every single noun can be used this way. In fact, other nouns modify other nouns all the time:
an atom bomb
a release date
a sports magazine
Some people say that sports and chicken here are "adjectives" or "nouns acting as adjectives". These people are wrong. How do we know?
They don't inflect like adjectives:
They don't accept adverbs as modifiers:
very hot soup
*very chicken soup
They often don't function as predicative complements:
It was a dangerous bomb.
It was an atom bomb.
The bomb seemed dangerous.
*The bomb seemed atom.
He made the bomb dangerous.
*He made the bomb atom.
They don't function postpositively:
This is hot soup.
This is chicken soup.
This soup is something hot.
*This soup is something chicken.
Attributive nouns are sometimes marked for number:
a sports magazine
a customs officer
a soft drinks manufacturer
the heavy chemicals industry
The Parks Department
Most attributive nouns are unmarked for number:
a trouser press
a trousers press (nonstandard?)
But adjectives are never so marked:
*a hots soup
*several beautifuls walls
In short, nouns remain noun-like when used attributively. Nouns don't suddenly become adjectives or adjective-like when you make use of one of their basic functions. Their main similarity to adjectives in this position is in being unmarked for number, but even there we can see a difference, and one that's widening over time—the plural attributive construction is becoming more and more common.
Sometimes attributive nouns are reanalyzed as adjectives. For example, in recent decades people have started to say funner, funnest, and really fun. For these speakers, fun really is a full-fledged adjective, and we can tell because it displays the characteristics associated with one!
But for the most part, there are clear differences between attributive nouns and adjectives, so there's very little motivation to use the word "adjective" in describing this sort of usage.