Phone cameras have improved over time, that's a fact. But manufacturers don't always improve them in the same way. Megapixels aren't everything and a large noisy shot is usually worse than a smaller, cleaner one. Over at PetaPixel, Sven Skafisk decided to chart the changes over the years, starting all the way from 2008 to now.
It's not the most in-depth analysis, but Skafisk's article does reveal some interesting trends. As mentioned, megapixels have gone up, but depending on who you go with, the difference can be has high as eight megapixels:
The 20+ megapixel phones are the Sony Xperia series. Because of the added light-gathering of larger pixels, there are still flagship phones made today with lower resolution, such as the 12.3 megapixel Google Pixel.
Aperture size is the other big one and while they've gotten better, it's going to be hard to keep up the pace:
I expect apertures to continue to get brighter over time, probably settling around f/1.4, but possibly as bright as f/1.2 or f/1.0 if the engineers can pull it off.
One observation has nothing to do with the cameras themselves, but rather, the number of them. We started out with one and up until recently, two was pretty much expected from any mid-to-high tier device.
But in 2016, we hit three and no, they're not sticking them on the side, but rather providing improved shooting options by pairing the rear camera with a focally-different buddy.
Skafisk includes a few graphs in his piece, which you can check out below.