About thirty years ago, I played through several of the Dragonlance modules, usising the book characters . My regular group became very frustrated , as the illusion of player agency was poorly maintained . Every time we ran into Fewmaster Toad, we killed him, and the blasted hobgoblin would be waiting for us in the next chapter, the only proof of our actions being the small amount of xp we had earned for killing him.
Plot protection is annoying. If something has a stat line, it can be killed. There are of course well written exceptions to this; Art of the Genre’s Folio adventures have set encounters that respawn, but remember your last interaction with them.
The first system I evr heard of that had a mechanic for effects of PC actions on the world at large was Mayfair Games Underground, I never played it, but I liked the idea of the quality of life of the common folk being affected by the actions of the heroes.
+Peter V.Dell’Orto discusses the independent progression of the game world frequently in his excellent blog Dungeon Fantastic. From his GURPS Dungeon Fantasy retooling of B2, where inhabitants of the caves not bashed by his PC’s joined up with the BBEG as undead minions, to the looming overpopulation of orcs in Felltower, created by PC negligence and collusion . The orc issue became so dangerous, that the PC’s risked a TPK by an army of trolls rather than face them directly .
The next time his players were in a situation where they only partially cleared an area of hostiles, they knew that would be the last time that the enemy would be that unprepared , that the fight would only be in easy mode one time.
Recently, fellow gamer +StephanieBryant proposed a setting where not getting it right progressively made it things harderon the PC’s, and I have begun to illustrate this for her.