The obvious solution is to use a Tester to drop those features entirely, if you don't want them included in the route.
Like Dan says, there isn't any tool to ignore a specific feature in the ShortestPathFinder. You can set a cost, but if you do then FME no longer uses length as a measure.
So, what you have to do is use a LengthCalculator to calculate the length of all features and use that as the cost attribute in the ShortestPathFinder.
For features with stairs=true, you could then apply a multiplier value to increase the cost before finding the path.
In fact, your flag could actually be a cost multiplier rather than a single boolean value.
So the actual cost is length * stairs * ramps * etc
The field could actually even be the number of staircases (+1) so a path with length 50 metres, 2 stairs, 1 ramp, and 0 crosswalks would be: 50 * 3 * 2 * 1 = 300
So you aren't absolutely excluding those paths, but increasing the cost quite heavily.