I just had some new Goodyear Viva 2 tires (235/75 15) installed at Walmart. The recommended tire pressures according to the manufacturer's sticker in the door jam is front 24 pounds rear 20 pounds or for full load front 28 pounds rear 24 pounds. I left Walmart with 40 pounds in each tire courtesy of the kid who installed them. I brought the pressures down to 29 front and 25 rear and the ride is MUCH better. Now the question - I know that 40 pounds was ridiculus, but running tires in the range of 20 to 24 pounds these days seems way low. All that stuff about the heavy SUVs running on slightly underinflated tires and having blowouts etc.

With tires that are produced today, nearly 30 years after Cadillac wrote the spec in the door jam, should I still adhere to that seemingly low pressure with modern tires? Is it safe? Thanks.

I'd stick with Caddy's recommended tire pressures. Tire technology may have changed but tire pressure/ ride hasn't. Run a little more and you may get a negligable mileage increase. If the ride is acceptable, stick with it.
Overinflating the tires e.g. 40 lbs will wear then out prematurely.

my 78 eldo has been running 32,000 miles on the same 4 cooper tires

my air pressure was always kept at 32 psi plus/minus 2 pounds

door sticker recommended pressure seemed low to me also

range should be door sticker recommened up to psi printed on tire sidewall

all depends what ride you prefer and your vehicle load (people in car, gas level, luggage etc.)