It might not have kept slavery out of the enire Southwest, but it might have had a real effect, especially in Kentucky and Tennessee:
***
"While rejecting the notion that Jefferson's free-soil clause would have had no effect, I do not mean to suggest that the clause would necessarily have prevented slavery from becoming firmly established anywhere in the Southwest. It would most likely have affected the northernmost and most mountainous portions of the Old Southwest, in which slavery never became as widely and firmly established as it did in Alabama and Mississippi. Kentucky was not part of the original Virginia cession, and the Bluegrass State never went through a territorial stage, passing instead directly out of Virginia into full membership in the Union in 1792.184 North Carolina's cession of Tennessee did not take place until North Carolina ratified the Constitution in 1789, at which point Congress took over administering the Territory South of the Ohio, leading eventually to Tennessee's admission in 1796.185 Regarding the remainder of the Old Southwest, it is all but certain that South Carolina and Georgia never would have made their cessions had Jefferson's provision remained in force, and even without an anti-slavery provision in place, conflicting Spanish claims, Indian wars, and complex Georgia politics involving various factions of well connected speculators with conflicting claims to Indian lands in the Yazoo delayed establishment of the Mississippi Territory (comprising the future states of Mississippi and Alabama) until 1798.186 At that time, Congress considered but rejected legislation that would have prohibited slavery in the new territory.187
"For the future states of Kentucky and Tennessee, however, approval of Jefferson''s anti-slavery clause in 1784 would have created a period of substantial uncertainty, and any uncertainty worked against the immigration of slaveholders. Demographic history prior to the first federal census of 1790 is inexact, but even in 1790 Tennessee (The United States Territory South of the Ohio) had a black population of only 10.6% (of whom 90.4% were enslaved), similar to New York's (7.6%, of whom 82.1% were enslaved) or New Jersey's (7.7 %, of whom 80.5% were enslaved), where slave owners lacked sufficient clout to prevent emancipation by political means.188 Whether slave owners would have streamed in to Tennessee in the 1790s with emancipation scheduled for 1800 is open to doubt. While Kentucky's black population for the 1790 census (conducted in the district which was then still part of Virginia) was already 17% (99.1% of whom were enslaved),189 it is questionable whether slaveowners would have risked establishing themselves in the region after 1784 with Jefferson's clause and its 1800 deadline looming over all territories to be ceded.190 This marginal uncertainty may have been determinative, and even in 1792, state constitutional sanction of slavery was only achieved after a hard fight in the convention.191 Immigrants into Kentucky between 1784 and 1792 could not have foreseen that Kentucky would never pass into territorial status, or that they would win constitutionalization of slavery at the time of statehood, even if the eventual separation of the region from Virginia was expected by the time of the Territorial Governance Act. All this is of course hypothetical, but while I cannot show that Jefferson's provision would have reduced the eventual number of slave states, neither can Finkelman show that it would not have..."
William G. Merkel, "Jefferson's Failed Anti-Slavery Proviso of 1784 and the Nascence of Free Soil Constitutionalism"
http://scholarship.shu.edu/cgi/viewcontent.cgi?article=1115&context=shlr
All the same, one must remember that Jefferson's proviso could not have prevented states from adopting slavery once they were admitted into the Union. Indeed, even Illinois seriously considered legalizing slavery in the 1820s--and Kentucky and Tennessee were of course to the south of Illinois Still, even in OTL there was serious emancipationist strength in the first Kentucky constitutional convention; see my post at
https://www.alternatehistory.com/forum/threads/kentucky-decides-against-slavery-1792.380040/ and it would presumabl have been greater in this ATL. As Merkel notes, people who moved into the Kentucky country in the 1780's would not
know that the area would become a state without going through a territorial period when slavery was prohibited. (That it would eventually separate from Virginia was widely anticipated.) Therefore, there might be fewer slaveholders migrating there.
The one thing of which I am pretty sure is that it is
not going to stop slavery in the Gulf states.
