I think if it were as simple as "remember the RNG seed", game developers would do it every time. But determinism also means, for instance, running the physics engine at a deterministic timestep regardless of the frame rate, to avoid differences in accumulated error or collision detection. And that's something that needs designing in from day one.

Thank you for still prioritizing it.

Mostly because determinism is an all or nothing proposition. Either EVERYTHING in game logic is perfectly deterministic and isolated from everything else, or it pretty much as if nothing was. So if you want to commit to determinism, you have to be constantly vigilant and debugging these maddening types of bugs. Whether this investment is worth it or not is up to each dev.

Sometimes you can find small areas of the game that can be deterministic and worth it. In a basketball game I worked on in the 90s, I designed the ball physics to be deterministic (running at 100hz). The moment the ball left the player hands it ran deterministically; we knew if it was going to hit the shot and if not, where the rebound would go to.

Determinism isn't essential to achieve record/playback experiences. You could just record the transform of the player at some fixed interval and then replay it with interpolation. This is arguably more "deterministic" in a strategic sense of shipping a viable product.

Probably (armchair HN reader, not a game developer here) due to dealing with multiplayer latency and / or performance / multithreading / scalability.