[theultdev]

That's your own prejudice coming out. The word boy may have a negative connotation in some usages, but in this case it's referring to literal boys.

The term was used for family and community farms, not for large ranches / plantations.

The fathers and sons would tend to a small head of cattle used to feed their family and the community. They did not and could not afford to own slaves.

Plus the term was used well after Britain outlawed slavery.

[tcskeptic]

I think the point being made is that it seems plausible that a term used in Britain to refer to literal boys was repurposed later in a different context to be a demeaning way to refer to an adult man working with cattle in the US.

[theultdev]

It might be plausible if any evidence was presented, otherwise it's very loose speculation.

Many of the cowhands working side by side vaqueros were literal boys (15-17) and very young men.

In Texas it's very common to call a young man a boy.

Currently it's a baseless accusation that it was a derogatory term, even moreso that it's racial.

It's never been mentioned as such until now, well past the high days of cowboys.