I agree that the above Strawman doesn't happen. I didn't say trivialize. I said makes the encounters easier, and invalidates the special abilities of the monsters.
No, you didn't use the world "trivialize". You said that, in the case of trolls, it would reduce the CR by 2 (from 5 to 3, in that case.)
But go use the official rules for challenge difficulty calculation, and see what happens when you change the CR from 5 to 3. In the few examples I tried just now, it reduced "Hard" encounters to "Easy" and "Medium" encounters to...wait for it..."
Trivial".
And it doesn't
invalidate the special abilities. Those abilities still work. In the case of trolls, if the go-to attack of one of the heroes is fire (or acid), the ability is mostly irrelevant, anyway, because they are going to be using fire (or acid) regardless of whether they know they have to. But let's say none of the casters chose a fire cantrip, which happens. What now? Maybe they have 1st level or higher fire spells, but now they have to start spending actual spell slots, every round, to counteract the ability. Or somebody has to use a torch for a weapon (possibly first using a turn to light one) which is going to sink anybody's damage. Or use alchemists fire, which is also a resource. And if there are
two trolls then two characters have to be on fire duty.
So even if you don't fumble around for a round or two figuring things out, troll regeneration still has to be dealt with. It's not "invalidated." If it were...and really this is the kicker...there would be no point in having trolls appear more than once per campaign under your rules, or
at all in default 5e (since there is no assumption anywhere in 5e that player knowledge is forbidden).
You keep claiming that abilities being secret is "baseline", or other words to that effect. Do you have
any evidence of this? Anything from MM or JC? We've already concluded there isn't a single reference anywhere in the game to player knowledge. Anything else you want to cite?
Now, the one thing I will grant you is that each player has one opportunity in their D&D career to encounter trolls for the first time, and that if one genuinely does not know about troll regeneration it can be fun to struggle with this. This is especially true if the whole table is new, but even if there's just one such player, I will typically not spoil the "secret" for them. (Curious how even new D&D players typically think of fire...less frequently acid...very, very quickly.).
The one time I saw a group of players (who, as I mentioned up-thread, were not new but hadn't played in a long time and had actually forgotten about trolls) really struggle with a solution, it was very nearly a TPK. Hard to believe that's a baseline assumption in the design of the game.