> A literacy test in early 20th century would already exclude a lot of poor people by definition.

That's actually an empirical statement, it wouldn't be by definition. (Unless by poor you mean "can't read", then it would be by definition.)

I observed that "by definition" is on its way to becoming one of these emphasis phrases, like "literally", no longer literally meaning what they used to.

Yes. Though even in its literal meaning 'by definition' is a weaker statement than 'by evidence'. At least in any argument outside of mathematics.

Most of the time the 'by definition' argument just gives you a True Scotsmen.

Take the classic: all men are mortal (by definition in Ancient Greek), Socrates is a men, thus Socrates is mortal.

As an empiric argument, this would make sense. But the Greeks tried to reach 100% certainty. Mortality was included in their definition of a human (otherwise you'd be at least a semi-God or so.) Of course, that just shifts the uncertainty to the second part: we can not be sure that Socrates is indeed human by that definition, until he actually keels over.

The literacy test was to gain citizenship after you had already been granted permanent residency, so while they wouldn't be eligible to vote in federal elections, illiterate people would still enjoy almost the benefits and rights of any other US person. Almost anyone can learn to read, it's not that hard, almost every 5 year old learns to do it. If an adult in the early 1900's wanted to be a citizen, but were blocked by a basic literacy test and still didn't learn to read it's because they weren't interested in putting the effort into it (there are people with severe disabilities, that is a totally different situation and i'm not addressing that). Of course it's way easier to be born rich, but if you think someone who is poor but motivated can't learn to read you are insulting their intelligence.

The literacy test was a barrier to entering the country, not to citizenship. https://history.state.gov/milestones/1921-1936/immigration-a...

And while that one didn't have a very large effect, the one that did block people from voting managed to filter out black people very well:

https://metro.co.uk/2017/09/20/could-you-pass-this-test-give...

To say that "anyone can learn to read, it's not that hard" because a 5-year-old can do it is disingenuous. It's been well established that children learn language and reading skills exponentially easier than adults due to the "flexibility" of children's minds that is lost with age. It can be nearly impossible for some adults, especially ones with no reading experience when they were young, to ever learn to read.