Head to either side of the street. (one or the other); Plant trees on either side of the street. (both)
It's yet to be found. (not found); Yet it's found. (found)
They're all but forgotten. (nearly); They're known but to God. (only)
It's yet to be found. (not found); Yet it's found. (found)
They're all but forgotten. (nearly); They're known but to God. (only)
I guess I am just unable to parse the words to piece together such a meaning. In my mind, there are two ways to interpret it: "this subject may be all kinds of things, but one thing it is not" (all but = everything except), or the opposite "this subject may be all kinds of things, but one thing it is for sure". And neither of these means "nearly" :(
You've walked all but the last step of the journey.