Hi, the reason behind moving away from the tagging method for these footways is that mapping them separately to the carriageway allows routing engines to follow the footway and crossing points more accurately, and in particular so that wheelchair routing profiles can make use of mapped features such as lowered kerbs on crossings.
Without this the route will simply follow roads rather than taking users to usable crossing points. If sidewalk tags are left in place at the same time as separate footway exists, then in certain circumstances the routing engine ignores the footway.
e.g. maps.openrouteservice.org