I’m going to keep this short as from what I’ve read on the topic the answer is likely “there’s ambiguity so it is turned off”.
I was running in to the problem when doing some programming exercises and noticed that when using one of the diagonal arrows (e.g. \searrow) to indicate a diagonal slice through the data the compiler threw an “invalid character “x” near column 1” .
By this point I had had success with arrows (e.g. \downarrow and \rightarrow) and decided to use a two non-diagonal arrows to indicate the diagonal instead. This now threw a “x” is not a binary operator error".
Is there a reason behind these decisions that I am just unaware of, as being able to define the direction in the variable name seems like the perfect use case for unicode characters.