Coal_ore is nearly 0.52 times less dense and iron_ore is about 0.64 times less dense in 18w versions compared to 1.12.2 and earlier versions. Density is measured relative to stone plus andesite plus granite plus diorite plus dirt in both versions. If this is intentional, please ignore. Otherwise, it makes survival significantly more challenging, especially early in the game. The plots show density as a function of height for both versions.
For reference, the ratio of the densities in 18w08b and 1.12.2 for r.0.0.mca seed 12345 are:
There is statistical noise in all of the measurements, most for diamond and lapis.
For what it is worth, I also analyzed the average patch or deposit size for coal_ore. The average size (number of coal_ore blocks) of a contiguous deposit of coal_ore in 18w08b is about 60% of the size of contiguous deposits in 1.12.2. There are also about 90% as many deposits in 18w08b as in 1.12.2. Basically coal_ore deposits in 18w08b are similar in size to iron_ore deposits in 1.12.2.
As of 18w20a/b nether quartz ore and lapis ore are still affected.