Is this retaining wall failing?
The wall may have been built that way but I doubt it, the wall may have shifted over the years and is stable now.

The only way to know is to drop a plumb bob from the upper decking just in front of the wall and measure the gap at top middle and bottom. Mark the spot or leave a nail or screw in that location and Mark the wall at the 3 points so you can measure again at a later date. If the wall is stable you will see no change in the measurements.

I would not expect to see any movement in a year, but probably would recheck a week after a big storm. There looks to be a drain but water may have caused the shift or belly in the past, prior to everything settling, and now the wall is stable.