The OP wrote “I wonder how the signers of this letter would feel if the Python conference was to be organized in a country where slavery was still legal.” I mentioned a country that is known for modern-day slavery yet draws foreigners, with the implication that it might not be as shocking for tech organizers as the OP thinks. For example, the country has recently been prominent in the OpenStreetMap community[0], but I have never seen the discussion go off into concern about slavery, and I wouldn’t expect a large outcry if one of the big international OSM conferences like SotM was organized there.
[0] https://www.trufi-association.org/projects/public-transport-...