A couple things on this:

- Not every setting is “commoners know Vampires are vulnerable to Radiant damage.” The DM, not the Players, are supposed to control how their setting works. If the Player is taking their knowledge of the MM into the game, with the excuse of “my character learned this from stories from the tavern”, well that’s just taking the DM’s ability to world build and crapping on it. The DM is the one who determines what stories are told in taverns, not the Players.

- The whole point (as far as I can tell), of having monsters with various resistances, immunities, and vulnerabilities, is to alter the challenge they would otherwise be to the characters, and add flavor to the creatures. That challenge is lessened if characters just know what to use or avoid in any given situation. As a DM, why use a Troll, if the Troll’s signature ability, it’s regeneration, is moot? You’re taking away part of what makes it a challenge. The flavor is moot, as the challenge is no longer “can the characters figure this out”, but rather, did the Players study the MM or adventure model enough?

- building on that, I believe encounters should be appropriately suited to the characters (not the Players). I, as a DM, can create encounters appropriate to the characters. I know their level and abilities and can build around that so each is relatively appropriate to some degree of easy, medium, difficult, deadly level, depending on what I’m going for.

I can’t build encounters based off of what Players know, as I don’t really have that insight. Further, my Players tend to have differing levels of experience, so what would be a good encounter built for one Player’s experience, might be a poor encounter for another’s.

- All in all, I feel if a DM is going to put the time and energy into world building and encounter designing, the least a Player can do is not try to undermine that effort. Doing so is just disrespectful to the effort the DM has put in.

Now, certainly, using the knowledge that fire stops a Troll’s Regeneration, is not the same level of a Player knowing that there’s a secret door in a certain the room, because they studied the module the DM is running the night before. But they are both examples of using Player knowledge to undermine the challenge created for the characters.