I'll caveat again. I have fostered sighthounds and I didn't let them off lead.
The original question was about letting the dog off lead. When I've got a sighthound that's what secure fields are for. If it's a sighthound or has a lot of sighthound in it then it's too risky for everyone.
I've also fostered lots of dogs that I was told had no recall and it was pointless trying.
I got most of them as safe as you can get in terms of rock solid recall and if I couldn't they stayed on a held or dropped long line. As a condition of their adoption. It's a small local rescue and I can have dogs for up to 5 months before they are rehomable and we stay in touch with their adopters. For context we've realised 24 dogs in 2 years.
It's a long, sometimes boring, repetitive process. I don't train it so much as always train it. Most people don't have the time or patience. Can't be done a hour a week in a secure field. It's starts in the house and really never ends.
There are some breeds that are easier. Labs, GSD, collie mixes. But I've had all sorts and lots that it was anyone's guess.
I'm often the person on the end of the lead of the fear reactive dog so I make it a mission to make sure I'm never the person shouting "don't worry he's friendly" to an on lead dog.
As far as I'm concerned it's the most important thing you can teach and I put the hours in.
I'm not a trainer, but I have had the chance to learn from a lot of good ones through the rescue. I get dogs that have been terribly mistreated or abandoned and we have not history.
So it's my job (volunteer job) to learn and apply as much as I can. These are dogs that have often been chucked out because they were "badly behaved". They weren't, they were badly trained.