There are many questions in the forum about routing problems caused by barriers, often with missing access tag.
https://forum.kurviger.de/search?expanded=true&q=Barrier
In several threads there is also a hint that other route planners can plan the desired road - that made me thinking about possible improvement of the Kurviger routing strategy.
I think it is not useful just to route through (closed?) barriers without any notification, but perhaps barriers could be handled in the same way as potentially closed roads?
An other idea: If a barrier has no access tag set, assume “access=yes” and route through it.
What do you think?
Do you have experiences on the “opposite” (there is a barrier where it is really not possible to ride through)?
Even some that use OSM data?? Then that would mean they operate under a “default = yes” rule as you propose, very interesting!
I think the main problem is that the “default = no” operation as Kurviger does it now simply is the less risky behaviour. A few forum posts here and there is manageable, more than 1-star reviews because we sent somebody through a gate and off a cliff 
But very interesting proposal nonetheless, I hope the developers can chime in here with their experience of what the default usually is
Yes - e.g. Calmoto does. If you plan the route mentioned in Route ĂĽber unbefestigte Strasse - #2 by rumbrummer, the route is planned through the barrier (without any notification):
Yes, that is indeed planned, but at least right now not yet possible as this requires some serious technical changes. There are technical limitations that makes this really tricky 
.
This has been implemented, also see here: [Routing Server] Update to GraphHopper 5.3 / Unblock Gates