Several people in my lineage were lefties. Born turn of the century or before.
Mother and Brother as well.
They had tales of having to hold rocks or coal in their left hands or actually have the tied behind their backs in order to become righties.
Didn't work and left bad memories.
In my service time, 60s, lefties were required to shoot right handed on training ranges.
I have no memory of how that worked.
I never heard anything about "weeding out" lefties. It never happened in my experience and I don't believe it ever did significantly.
They may be sinister or mutants, but still useful in general.
Ones in my family seemed pretty normal-mostly.