How to call left-handed people who were not forced to change to writing right-handed? Unrelearnt left-handers? Is there any optimal name?
There is no word in common use that means "a left-handed person who was not forced to change to his right hand for writing".
You could say:
He writes left-handed.
He is naturally left-handed.
Handedness so-called is not really binary. You can write left-handed but use a knife and fork right-handed (and chopsticks left-handed). If thrown a ball you might naturally use your left hand to catch it even if you do not write with your left hand. Which hand you use for intimate personal hygiene is another matter too.
It follows that the category 'left-handed people' is not well-defined, and that it is probably not a good idea to look for a word to describe such an ill-defined group. Left-handedness, whatever it might mean, has been used as grounds to persecute people, even small children. Take care.
As far as I know in the USA, impelled right-handedness is falling out of favor and may be seen as an archaic practice, especially in secular education. Therefore there is no common term to describe left-hand people that's specifically related to not being forced to change their dominance.
I am left-handed. I've been commonly described as a "lefty".
"Southpaw" is also a term that can be used for lefties that is named after the boxing stance used by left hand dominant boxers.