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Just noticed this today, it is still broken and makes 2 bike parking spots unusable due to not being secure

Rack is still not attached at one end. Depending how the bike is locked, you would just have to slide the lock off the loose end of the rack to take it.
Perfect might be the enemy of the good for a repair on this custom rack.
Maybe instead of replacing the original concealed mounting hardware you can set a short post in the sidewalk where the rack touches the ground, drill a hole sideways through the loose rack end and bolt it with something tamperproof to the post. Basically the same job as setting a new no-parking sign.

I reviewed these custom rack designs years ago when on the bike committee and for this one I worried mainly that nobody would water the plants. Didn't think I'd be trying years later just to get it attached to the ground...

Still broken. Just checked.