I'd thought you cannot end a sentence with a noun. Rather:
noun + だ。
noun + です。
noun + である。
noun + であります。
noun + でございます。
But in a movie dialogue, I heard: "大切なのは、心よ。"
Ending a sentence with a noun sounds weird. In every situation, I add "です" (or でございます). But, what is "よ"? a 助詞？Does it effect whether to voice the "です"?
I think it's the usual 終助詞「よ」 (sentence-final particle よ). What I've read is that it attaches directly to nouns in so-called 女性語 (feminine speech). I think it's often used in stereotyped dialogue in fiction, so it's also an example of 役割語 (role words)--though I don't mean to imply that it's only used in fiction, or only by women for that matter!
Wikipedia's article for 女性語 says:
よ 【終助詞】 「わ」と並んで多用される女性語。「ね」と同様、「そうよ」「きれいよ」など体言に直接続く形式は女性的とされる。ただし、「よ」を下降調で伸ばしながら発音すると、「あっしは神田の生まれよォ」のような男性的な表現になる。
I've bolded the relevant part for emphasis, which I'll translate loosely here:
In the same way that ね attaches directly to uninflected words, so can よ, as in the examples 「そうよ」 and 「きれいよ」. This use of よ is considered feminine.
The dictionary 大辞泉 also talks about this use of よ:
現代語では、終止形に付く場合、男性語としてはその終止形に直に下接するが、女性語では「のよ」「わよ」「ことよ」「てよ」などの形で、また、名詞、形容動詞の語幹に付いて、用いられることが多い。
Again, I've bolded the relevant part for emphasis, and I'll do my best to translate it:
In women's language, it is often used in forms such as 「のよ」「わよ」「ことよ」「てよ」, as well as attaching directly to nouns, including adjectival nouns.
Note: I changed the translation slightly at the end to say "adjectival noun" in place of 形容動詞の語幹, because the root of a 形容動詞 is simply called a na-adjective in the grammar that is usually taught to non-native speakers. The dictionary says it this way because, in traditional Japanese grammar, the combination of a na-adjective plus an inflectional ending such as だ or な is considered a single word.
At the end of your question, you ask if です is omitted before よ. I think you can analyze it that way, sure. Except I'd usually assume it's だ that's omitted, not です.
