I've got a bound copy of The Gentleman's Magazine for the Year 1811 and there are some quite horrific entries in it dealing with slavery. There is one which I think is relevant to the topic under discussion. It appears in the July 1811 edition, under the heading:

"TRIAL AND EXECUTION OF The Hon. A.W. Hodge at Tortola for the Murder of his Negro Slave Prosper."

I would, however, like to preface it with a lengthy preamble. Like Antigua, Tortola is in the British West Indies. And also like Antigua, it formerly relied on sugar and slaves for its prosperity. According to the official Tortola webpage, on this island: "They cultivated a profitable sugar industry with sprawling plantations and imported African slaves. With the abolishment of slavery in 1838, however, came the demise of the plantation economy; as a result, many European settlers returned home."

Anyway, Austen could easily have read the following article in the Gentleman's magazine. It is rather graphic, so I'm adding a warning here. But as this is the 'Life and Times' Board, and as the Gentleman's Magazine was considered perfectly acceptable genteel reading of the day, I'm chancing quoting from the article verbatum. (The Gentleman's Magazine was, after all, the publication which carried Austen's obituary.)

The premise of the incident being reported is this. For failing to stop a mango from falling from a tree, plantation owner Hodge fined his slave Prosper six shillings and when the slave could only raise half this sum, the magazine reports that:

"Prosper was flogged for upwards of an hour, receiving more than 100 lashes, and threatened by his master, that if he did not bring the remaining 3s on the next day , the flogging should be repeated; that the next day he was tied to a tree, and flogged for such a length of time, with the thong of a whip doubled, that his head fell back and that he could cry out no more."

Prosper is subsequently found dead in the slave quarters in the
following condition:

"He was found there dead, and in a state of putrefaction, some days afterwards; that crawlers were in his wounds, and not a piece of black flesh was to be seen on the hinder part of his body where he had been flogged."

Other witnesses are reported testifying to the fact that Hodge had killed four other slaves for similar misdemeanours, including his cook Margaret who died when he, "poured boiling water down her throat."

Hodge is also quoted as saying that, "If the work of the estate was not done, he was satisfied if he heard the sound of the whip."

With graphic reports like these appearing in publications with such wide readership as The Gentleman's Magazine, it is inconceivable to me that Austen could have been unaware of the particulars of slavery. How she felt about it herself we can never know. I suspect that she adopted a rather pragmatic attitude. No time to look up the actual quotation, but didn't she reply of the soldiers dying in war something to the effect that: "What a pity that so many young men are dying and what a blessing that one knows none of them."

But as for Sir Thomas and the mention of his Antigua sugar holding, I tend to agree with Linden, who I think is arguing (correct me if I am wrong) that Austen's audience would have automatically assumed that this meant he was a slave owner, unless specifically told otherwise.