Oh I should’ve said chattel slavery, but still abolitionism older than you imply. In any case, however, it was very much a thing by the time of the founding of the US and a few states had banned slavery even before the American war of independence. Even if we ignore the whole “not giving them their freedom despite the will of his father in law explicitly demanding it” part, someone like Washington could not have been ignorant of abolitionist thought except willingly.