Around 60% of people are tone deaf and these people will never be able to sing in tune.
The first thing I did with people was to play a note on the piano and see if they could match it and if they didn't I knew I was in trouble.
If you can hit that note immediately and other notes that I play then you're in with a chance.
I have made tone deaf people sound good in a recording studio by having them sing one line at a time over and over again and adding everything together.
That's not true. It's actually only around 3 or 4% of the population who are tone deaf. Anyone else can be taught to hit pitch with the right training.
They might not have a good quality to their voice in terms of timbre, but they can be taught to hit pitch accurately.