When Kenny dies, he doesn't get reborn through his mother giving birth to him again. While Carol McCormick is shown to still be alive, she definitely hit menopause, meaning Kenny is gone for good this time.
Senior healthcare has become so advanced that retirement homes are overcrowded with elderly people who aren't allowed to die (as any of them dying for any reason would cause the entire country to go into lockdown) — partially becoming Truth in Television with how baby boomers are flooding long-term care facilities thanks to modern medicine keeping them alive longer than ever. Many developed nations are facing a crisis of aging, as they have to support an increasingly elderly population with fewer young people to provide taxes.
During the sequence where everyone is being vaccinated for the new COVID-19 virus, LeslieMeyers can be briefly seen in the crowd. Considered the sorry state South Park is in, one can assume the worst possible thing that the ads have done.
Timmy is absent for the class reunion. He's been hinted to have hydrocephalus and people with said condition don't live for too long. Though it's subverted when Timmy finally appears in the next special alive and well, his head smaller and walking with a cane, implying that medical procedures have advanced enough in the future for Timmy to either cure his conditions, or at least make them less severe.