Have an interesting topic. Today I had a first run on Zwift. I know from the other topics here, that Zwift does not support inclination data at all from any device - it just reads speed from the treadmill and eventually cadence from foot pod. Now, during the today’s run I changed incline for a moment and later after export to Strava and Garmin I saw something on elevation chart. So i made a test - started a run on Zwift, increased incline after 200 m, then reduced to 0, then increased, decreased, and so on, increasing and decreasing speed each time. After import do Garmin Connect, that is what I saw…
It seems, Zwift fully supports inclination readout and it is 100% accurate 
. Maybe it would be some sort of idea to include this information in the Zwift running workout interface - if there is some incline on the route and there is a setting on the treadmill of 0%, maybe it should suggest to set it higher, in the same manner as “spin faster” and “more power” in bike mode. :))