When I was a young kid we were out of toothpaste and some adult suggested using soap. I remember it wasn't very nice, but that's all.
For the sake of science, I just repeated that, so as taught in school, the scientific method.
experiment: brushing teeth with soap instead of toothpaste.
apparatus: toothbrush, soap (sainsbury's cheapo), teeth, darwin-award level of stupidity.
result: surprisingly inoffensive. Almost no taste of soap unless the suds reached the very back of your tongue. Considerably foamier than toothpaste!
conclusion: for this brand of soap at least, an acceptable substitute. Minty soap would be better.
Note that toothpaste actually contains a very fine abrasive which soap lacks. Also laundry soap (came in thick green bars IIRC) was probably far worse.