Stringer install

there are many ways to connect stringers to decks. the plywood isnt sufficient but some 2x4s probably could have been added to the bottom of the joists to make something solid to attach the stringers to. its not really a cut and dry issue. OSHA would probably say something like: attachment needs to deflect no more than one inch with 200 lbs of direct pressure over entire width of stairway or 6’ if stairway exceeds 6’ width

No, The plywood was obviously their cheap way out.
It appears as though they used a 2x6 or a 2x8 frame.
Instaed of using 2x10 where the stairs go, so more of the first rise has more support.
Also every stringer should have a 1-1/2" X 18" 12ga. stair strap on it, using #10 galvanized joist nails.
Almost all states fail a 2x4 under the frame. There is very little support that way and
attaching hangers to that floating 2x4 is pointless.

Thank you all. Paul

Here’s a good video that may be helpful: