Ashes! NO you should definitely NOT use drywall/plaster board in your soil!! It is gypsum as well as all kinds of petroleum and chemicals NOT good for soil, especially clay or soil you want to grow edibles!! Do you remember the site you read or where you got THAT information? Please send it if you are able.
Here is the absolute truth; the ONLY way to improve ANY soil is to add DECOMPOSED ORGANIC MATTER. The key word is decomposed, already been broken down by decomposers. That matter is what 'feeds' the micro and macro organisms in any soil. These organisms eat this stuff poop it out and mix this stuff into the soil for you. As long as you've got only decomposed organic mulch it is used immediately and these organisms go out to tell their friends and multiply like crazy. Plants and these organisms need each other. This decomposed organic matter as well as the pooped out stuff adds the necessary TILTH for porosity and holding air for plant's health. No matter if you've got sand, silt, loam, sandy loam, clay or clayey loam, etc.
Think about how concrete is made; sand, gravel, gypsum, lime, water and CLAY. Then rotation and mixing is added. Clay is a very cool soil. Most of my experience has been with clay; blue clay, caliche clay and all of it all I used was DECOMPOSED organic matter...and fertilizers as needed for the particular plants planted. I add lime ONLY when I want to raise the pH for certain plants and that is ONLY after viable tests. The only time I add elemental sulfur is when I want to lower the pH ONLY when I know the original pH for a particular plant or crop.
Clay is the tiniest of the little rocks that make a soil. They are also FLAT. This gives them an electrostatic charge that HOLDS the particles together, necessary for concrete. You never want to ever rototill and manipulate clay, especially WET. Just dig and dump, slightly chop...look up 'double digging and raised beds without any construction'
That is it! I'll never add anything else...double dig your soil and throw in DECOMPOSED ORGANIC MATTER, (not just organic matter, decomposed or the immediate decomposers will gobble up any available nitrogen to do their work and the important micro and macro organisms go dormant until the non decomposed organic matter is decomposed so that it is then available to the soil organisms for food. Raise your beds. Provide trenches for drainage. And often just add decomposed organic matter right to the top of your soil. The organisms come up and eat it, go back into the soil profile to poop it out continually improving your soil!
Take the drywall to the dump in the construction debris section!