Ikea furniture and any flat pack furniture can usually be taken apart just fine. Just reverse the instructions. I've done it multiple times, especially when I had to fit a desk with a hutch in the back seat of my sedan. I guess for most people, it's just not worth the time to take apart all their furniture at once
A lot of IKEA stuff I've bought involves one-way assembly -- pounding things together with a mallet. See the EKET cabinets for example. If you reverse it by naively prying them apart, it'll strip the wood.
Or take the backing to any BILLY bookshelf -- it's held in place with ~30 tiny nails. It's virtually impossible to pry the backing and nails off without destroying the backing in the process.
Etc. etc. Yes, some of the pieces can be disassembled, if the assembly consists entirely of cam locks, dowels and screws. But it's certainly not designed for it.
In my experience, the problem isn't just reverse the instructions, but rather the furniture ins't as sturdy as it was the first time I built it.
Anecdotally, I just moved apparently. The the last two years my Malm bed was fantastic. Never made a noise no matter what I did on the bed. I could lay, kneel, have my dog run zoomies on it, and it was great. Then I took it apart to move apartments and you would have thought a banshee lived in the joints. Sitting, touching, it didnt matter that thing howled. We tried a few online solutions before doing the ultimate solution: buying a new frame.
