If you want to record a ton of boolean values you should look into sparse bitsets like roaring bitmaps. You can encode a ton of information that way and serdes is trivial.
Then when I check if NPC x knows about EVENT y I check who he met between time of the event and now and look at importance (witnessed murder would be high) and if there's a path for the information to pass - then he knows.
But it's too much work to do in every "tick" of the quest process. It should only be checked when something changed (someone who knows meets someone else).
It became too complicated system for just a few usages and some background gossip. But I think for a big RPG it could be great. Would allow more reactive world and more interesting quest design.
I think the main reason AAA games don't do this is voice overs. If we had good emotional speech synthesis a lot of new possibilities would open. So indies have an advantage here because text-only is still accepted.
I think where such a system would shine is in background banter. The player has to observe the NPCs telling each other these facts so he can appreciate it. That also opens the door for the player stepping in to prevent the spread of information. With the right setting that could be a major game mechanic