This is more of a historical question: who invented the notation $\perp \!\!\! \perp$ for denoting (conditional) independence?
I have often seen it associated with AP Dawid 1979, "Conditional Independence in Statistical Theory." For example page 373 of these notes. I have no idea if Dawid actually invented it or popularized it.
Update 8/17/19: According to Wikipedia, the symbol was introduced by Elfving: "Elfving introduced the statistical symbol for probabilistic independence ⊥⊥, which is a stronger condition than orthogonality ⊥, by the 1950s."