interesting, it seems like GraphHopper does not use piers, this seems like a bug to me.
One example is shown here. GraphHopper should use the pier. Piers seem to be enabled for foot routing by default. There are also no restrictions mapped for this pier. BTW: I know this pier and yes, it should be accessible by foot and it is required to use it if you want to get to the ferry.
I will create a PR for GraphHopper to use piers, so you don't have to do this unless you want to :)..
You will need to change the FootFlagEncoder and FootFlagEncoderTest, since pier is not tagged as highway you need some special handling and cannot simply do it as the other highways, but like we handle
railway=platform for example.