Any where Biden got fewer than Hillary?
Sharkey and Issaquena in MS. In fact, Sharkey also gave Trump less votes in 2020 than it gave him in 2016...I guess it's just hemorrhaging people, though it might have also been because there was a stronger third-party performance in the county in 2020 than 2016 for whatever reason.
I looked up Sharkey County's population data, and it lost 11% of its population between 2010 and 2020, on top of losing 25.3% of its population between 2000 and 2010, and it's got less than half of the population it did in 1970 (4,377 vs 8,937). It must be pretty depressing for people from areas like this to watch their communities decline on such a scale.
It's the same story in parts of Appalachia as well (i.e., Eastern Kentucky and West Virginia), and a lot of the Mississippi Delta. Sharkey County is far from unique - there are many other counties in the Appalachia/MS Delta that are extremely poor (Issaquena County, bordering Sharkey, is apparently America's poorest county by some measures) and unhealthy with low life expectancy and rapidly declining populations.
Here's an interesting map that shows when each county peaked in terms of population:
The counties present in each category can be broadly categorized with ease in terms of their socioeconomic development. The early peaking counties (green) are mostly rural areas that lost people moving to cities during the process of industrialization, which made a lot of farmhand jobs obsolete. Early 20th century peakers (orange) are similar in this regard, but it also includes a lot of Black Belt counties that lost people to the Great Migration and Plains counties that were decimated by the Dust Bowl and Great Depression. Late 20th century peakers (red) are a mix of areas that once depended on manufacturing or resource extraction jobs as well as some more agricultural areas and urban core counties that experienced white flight. Counties that peaked during the last decade or two (blue and dark blue) are those that have more recently begun to suffer from the general depletion of rural and small-town America. Lastly, counties that gained population in 2020 (black) are predominantly well-off metropolitan areas with good job availability, are popular retiree destinations, have universities, resorts, or other socioeconomic pull factors that draw in people from elsewhere (in the case of the Mormon Belt, population growth is also being driven by strong but slowing birth rates).
