J.F. Sebastian? He's suffering from a genetic condition that causes him to age prematurely (ironic, since he's a geneticist who designs replicants and simpler creatures). He failed the medical screening required for off-world living, and thus is confined to life on Earth.

I'm afraid I disagree with your claim that J.F. is "suffering serious cognitive decline" due to his life on Earth. To the contrary, his confinement on Earth is not the cause of his psysical decline, his physical condition (genetics) is the cause of his confinement.

more details: https://bladerunner.fandom.com/wiki/J.F._Sebastian

I think linsomniac is crossing wires between the film and PKD's original novel: in PKD's novel John Isidore, a radioactively damaged and sub-par IQ human, aids the fugitive androids. And in PKD's novel, the remaining residents on Earth wore lead codpieces to protect themselves from mutation.

The film differs in that J.F. Sebastian is a creative genius, not sub-par IQ, and yes his genetic condition is the proximate cause of his confinement on Earth: but the film clearly asks: what caused the condition? The film doesn't say but pretty much implies the same as PKD's novel [that one of Sebastian's parents or ancestors was radioactively damaged].

In general the film implies but does not lay out any of the (unnecessary and seriously tiresome) exposition from matchstick characters that PKD's novel did, and that makes it stronger. (Arguably except for Gaff, and especially the unicorn scene in the Director's Cut). Ditto, omitting Mercerism in place of a general existential malaise.

I might have misinterpreted something, but wasn't there an exposition piece from Isidore about how he could heal animals and humans before his abilities were burned out by exposure? It sounded like fantastical ravings of a diminished mind but maybe it was meant to highlight in his fantasy world his desire was to heal.