Route planners with route avoidance features
https://brouter.de/brouter-web/


Allows you to draw no-go areas.
Allows you to specify cost for ascents or descents. I’m not sure it’s possible to tell it to completely avoid streets above a certain incline. You could set the cost very high and set the cutoff to whatever you like. Disadvantage of this approach is that inclines below the cutoff are not considered for routing (i.e. it doesn’t try to avoid shallower inclines anymore).
Allows you to specify a plethora of other parameters and can be scripted.