You are arguably supposed to keep a full record of all subs in any game, but as you've obviously concluded, that's ridiculous in a roll-on-roll-off league game.
My RO/RO league cares about which players have played for the sake of cup and league medal eligibility, so all I realistically need to do is tick when they come on for the first time and make sure subs are wearing appropriate bibs/tracksuits so it's obvious who's who if anything kicks off. The difference between and player and a sub receiving a red card is significant, but a lattice diagram of who's on the field when isn't necessary to get that right.
As others have pointed out, any match where it might go to penalties (KFTPM) is the exception to this, at which point you need to make sure you're ready to monitor who was on the pitch at the end of the game. As long as you have a way of doing this, you're fine - be that using a record or actively managing the benches at fully time.