Before getting into the history part of this question, I want to run through the basic genetics of handedness and show what they mean in practice. This will allow us to significantly narrow the types of past events that could conceivably affect today's percentage of biologically left-handed people.

There's a variety of strong evidence that handedness is genetic, including the facts that identical (monozygotic) twins are more likely to have the same handedness than fraternal (dizygotic) twins and children with left-handed biological parents are more likely to be left-handed.

However, handedness can't be explained by the simplified genetics that we all learned in high school biology. Instead, it seems to be the result of multiple genes. The end result is that:

• A child with two right-handed parents will be left-handed around 9% of the time

• A child of one right-handed parent and one left-handed parent will be left-handed around 19% of the time

• A child of two left-handed parents will be left-handed around 26% of the time

This gives some interesting results; bear with me for a minute to see what I mean. Imagine that the entire world population consists of 1,000,000 people, of which 100,000 (10%) are left-handed. We'll call this group Generation A, and we'll say that they do a horrible thing: they kill all 100,000 lefties before any of them have children. To make things easy, I'm going to assume that everyone marries, handedness plays no role in spouse selection, and each couple has 2.5 children on average. Here's how the population breaks down over subsequent generations:

• Generation B: A total population of 1,125,000 people, with 101,250 left-handed people (9% of the population)

• Generation C: A total population of 1,406,250 people, with about 151,000 left-handed people (over 10% of the population)

Even though every left-handed person was removed at the beginning of our imaginary world, the population returned to the expected percentage of left-handed people within two generations, and wasn't all that far behind after a single generation. This is to be expected, given that 9% of children born to right-handed couples will be left-handed.

Given everything above, we're looking for actions and events within the past 2-3 generations that were widespread and extremely effective at preventing left-handed people from having children. On a global level, I think it's safe to say that nothing like this has happened in the past several generations. Even preventing ten million left-handed people from having children would have very little effect on the overall left-handed population one generation later. The Catholic Church may have frowned on left-handedness, but they were teaching people to pretend to be right-handed, not systematically hunting down and killing left-handed people by the tens of millions. While left-handedness has been discouraged quite recently in certain countries (and even certain areas of the US), I haven't been able to find anything systematic and dramatic enough to significantly change the rate at which left-handed children are born, and any less-serious discrimination affecting handedness would be too recent to discuss in /r/askhistorians.

Edit: A quick note about the science. While I reviewed scientific papers to make sure I had the basics down, I'm certainly not an expert and will gladly defer to anyone who is. That said, I feel fairly confident that the explanation above is a solid (simplified) explanation of the genetics behind handedness.