Earlier, Poland was cited as an example of a country where this assumption doesn’t hold. From my own vantage point in the U.S., segregated paths are common enough to have standard signs (below in English and Spanish), but the standard doesn’t require the bike lane to be on the left and the pedestrian lane to be on the right; it specifically allows the symbols to be swapped.
segregated=yes might also be appropriate for multi-use paths where pedestrians must keep off the path, sticking to the soft shoulder, if there are any cyclists nearby. It could be awkward for this key to imply a certain layout if we don’t have a very clear way to override that layout.