I used to keep notes on my own game and a couple of notes on opponents. To be honest though, it became too much like hard work so I stopped. Now I make mental notes during games which I share with team mates or I may on occasion record how each point is won during a game and share those with a team mate at the turn round eg opponent won 3 points with their forehand but you won 7 points from your opponents errors from their forehand side so forget the winner that wizzed past you and focus on exploiting their forehand weakness.

Players can also improve over time so a weakness at the start of the season may become a strength when you next play them. If I do scout players for team mates, it's usually done on the day when their opponents are playing other players so the information is fresh and relevant.

Ultimately, when the team I played for got promoted to the premier div, I found I had more success at the start of matches, by game 2 they were used to me and by game 3 they had worked me out and destroyed me. Good players are able to do that, they don't need notes, they work people out quickly during the game and probably they enjoy the challenge of doing that.