Rather than deleting the highway=* tag I suggest we use the lifecycle prefix https://wiki.openstreetmap.org/wiki/Lifecycle_prefix#Stages_of_decay per the guidance at https://wiki.openstreetmap.org/wiki/Australian_Tagging_Guidelines/Cycling_and_Foot_Paths#Closed/Illegal_Paths

For example disused:highway=path or abandoned:highway=path or demolished:highway=path or destroyed:highway=path depending on the state of the unsanctioned and illegally constructed mountain bike tracks.

