I’m looking up photos of restaurants 40+ years ago and struggling to find any obvious acoustic differences in their designs (I do notice carpet seems more prominent?) Do you have any examples of what they used to do better?

Booths, designs, and acoustic tile ceilings off the top of my head.

They went with easy to clean floors and took out the acoustic tiles leaving the ceilings and air handling systems bare and echoic.

In fancy buildings you also had a lot of decorative wood and molding breaking up the sound. And those embossed tin tiles, covered with a few layers of paint.

Obviously it varies widely by restaurant and location, but in general I'd agree with the statement that restaurants are a bit louder than they used to be. I'm talking about table service restaurants, rather than fast food. I think the reason is probably that real estate is more expensive now, so restaurants are trying to pack people closer together. Architectural styles are different as well, with spaces being more open, ceilings higher, and more hard surfaces (how many new restaurants have carpet?). There may be differences in people's behavior too, but I can't say that for sure.

For a while during covid, a place I would go to on occasion had full-height plexiglass dividers between each booth. It made such a huge difference in noise, I was sad when they got rid of them.

The people were well mannered, nearly every single one

Red Robin isn't exactly the type of restaurant I would think of when I want a quiet table - it is half a sport's bar after all.

If you go to a nicer restaurant, you will get a quiet table. It's the type of restaurant that matters - not so much acoustic design.

No quite the opposite. It has for a long time been the loudest place I knew of, but other places have narrowed the gap.