This was one of those TOP THREE MATCHES blanks, where they polled the audience and got the three most popular, written replies.
Reply first, to this topic, if you didn’t mind. Reply with your answer and see if you can find out what number one was. Then look at the answer in spoilers below and edit your reply to tell me if you’ve ever heard of that phrase before.
Number one was Is it soup yet?
What.
Ready. IMHO that’s the obvious answer.
“Ready” was the answer I came up with immediately as well.
And I’ve never heard of the spoilered phrase before.
My first thought was “ice”
The phrase was the punchline for a commercial for
Lipton’s Noodle Soup, a just-add-hot-water brand, like today’s ramen.
It wasn’t very good, especially compared to the real thing, but you didn’t have very long to wait for it.
Undoubtedly referring to the slogan in an ad campaign for Lipton Soup which aired frequently during the late '70s and early '80s. https://www.youtube.com/watch?v=ujaQg1e_PJU
I thought of Ready and Done, pretty much at the same time. I never even thought of soup and I’m positive I’ve actually said that before. Weird.
At my next to last job, I would have said “Friday.”
“Soup”
ETA: When I was a kid this was a euphemism for “Is dinner ready”. Yeah, I’m old.
But
Soup is Good Food
!
That was the competition’s slogan. 
Is it food yet?
Yes I think I remember hearing the phrase from the OP and just misremembered it. Haven’t heard it in a long time.
40+ years later, I still use the phrase when my wife is cooking a meal. I’m a nerd with a long memory. 
in
Best answer so far.
Now I need a new keyboard
That phrase was my response. I’m familiar enough with it that it’s the first thing that came to my mind. I’ve heard people say it, but only people older than me.
The only answer is ‘soup’. Strangely, I can’t find the commercial.
ETA: OK, there it is. I don’t know why my search didn’t find it.
Is your keyboard all sticky now?
For me, the most immediate answer comes from a local punk band a million years ago whose members would play random disconnected noise on their instruments while the frontman screamed “Is it jazz yet?” into the microphone. Alas, this is evidently far too obscure for The Google to clarify that long-lost memory.
Is it DONE yet?
Second place idea: is it SOUP yet?
(I wrote this without looking at the spoilered answer, or any of the thread past the OP)
ETA: And it looks like I got it (with my second answer)