According to the BBC, slit pupils are a sign of having "multifocal" lenses. Amongst other things, multifocal lenses allow animals to focus on different colors. Why would this be important to cats?

Drug use: Various drugs produce different effects on pupil diameter—stimulants such as cocaine and amphetamines tend to dilate, whereas opioids such as heroin tend to constrict. Neurological ...

Our eyes offer clues about our mental health. Changes in pupil size may signal stress or neurological issues. Mehezabin Dordi from Sir HN Reliance Foundation Hospital highlights this connection.