While using my circular to cut away rotted subfloor sheathing, I accidentally cut a thin kerf notch into the flat 2x4 top of my engineered floor truss joists. The cut is about 1/2 inch and very thin. How much risk am I assuming if I ignore this? There will be concrete and tile over this subfloor. See attached photo. That edge was made by my circular saw which I assume was set to incorrect depth.
Well, it doesn't matter how thick it is, but it does matter how deep it is. If you've only cut one or two trusses, and it's no more than 1/2" deep, I don't think you have a problem. It's extremely unlikely that it'll cause failure of the floor system in any substantial way.
However, since you have fairly easy access, I'd attach a repair member to the underside of that board, fastening it with construction adhesive and 3" gold screws (through the subfloor where necessary). Make it as long as can comfortably be fit in the space available.