Hi, as far as I know pawn sensing is not being developed anymore for some years now (from this post here by mieszko Pawn Sensing vs AI Perception - #3 by Answers.Archive). I never used pawn sensing, only AI perception, so I cannot really tell you which one to use, but since AI perception is the new system and still gets updates I would rather choose that.
However, I see that there is still pawn sensing in the current 5.1.1 version
As long as keeping pawn sensing in the engine does not cause any problems, I don’t really see any reason for it being removed, since some projects may use it and could then not update to a newer engine version.
To get an overview about AI in unreal engine you could watch this video here https://www.youtube.com/watch?v=iY1jnFvHgbE