Modern flash memory uses multiple voltage levels to cram two or more bits into each cell, in order to increase density.
This technique is not used in DRAM (perhaps because it would compromise speed?) but it turns out to be workable for ROM, indeed to have been used for microcode ROM several decades ago: http://www.righto.com/2018/09/two-bits-per-transistor-high-density.html?m=1
I get the impression from that article that after the 8087, the technique was not subsequently used. Has it been used in more recent times? If not, why not? Double density would seem to be a big win.