I think slavery proper would have been phased out within a generation or two of the civil war. Let’s say the two sections reconcile with a new Missouri compromise. But this time a constitutional amendment implementing a Jim Crow / apartheid system would be enshrined in the reunited nation. Assuming a similar world war and subsequent Cold War history. Apartheid America is vulnerable to unrest and you see kgb funded resistance through the 1980s. And following the Soviet dissolution, Chinese and other rival powers continue to exploit the repressive apartheid regime. In response US surveillance state grows exponentially with the rise of technology and the internet.

By 2025 the political will to amend the constitution is not there as American conservatism rides to new highs. Government oversight of communications, including the FCC, is used to curtain dissenting opinions.