It's in the wiki:
access= yes "The public has an official, legally-enshrined right of access; i.e., it's a right of way."
https://wiki.openstreetmap.org/wiki/Key:access#List_of_possible_values

The motor_vehicle=private tag which was there looks dubious to me.

I would suggest either access=destination (you can only use it to get there, but not use it as a through route/short cut) or access=permissive (no right of way, but nobody signed or physical restriction).

The bits of road behind ExCeL which are unadopted probably ought to have at least access=permissive + ownership=private

If there's a signed diversion of National Cycle Route 13 along Sandstone/Seagull while the dock side is closed in front of the ExCeL extension, they might need bicycle=permissive as well