"For example the Intel 8087 used two-bits-per-cell technology, and in 1980 was one of the first devices on the market to use multi-level ROM cells."
...and indeed I thought this article would have a mention of it. Certainly interested in any more exploration you can do about that.
This will be annoying if I try to visually read out the microcode values since I'll need to examine each transistor closely. It was quick to read out the constant ROM since I just needed to glance to see if a transistor was present or not.
Here's a photo of a 7x5 region of the microcode ROM to help the discussion. The transistors (representing bits) are where the vertical polysilicon lines cross the pinkish doped silicon. Note that the "neck" is full-width, narrowed, very narrow, or entirely gone (no transistor), corresponding to the 4 values.
Image link: https://photos.app.goo.gl/pUsZSs3rx45Ry7U1A
I do concur that this would be an interesting topic for another post.