1. Which settings are affected specifically
First and foremost and most important is the intensity of the braking. There is a standard value and to this is added small ranges up and down. In this way, each player will better control the distance and the road situation, understanding the behavior of the player in front, and thus more correctly choose the distance and speed. Now you can move at a safe distance behind the player, but if he uses maximum braking intensity and you are not aware of it, you will almost certainly have an accident, or keep a very long distance.
2. How do you notify players of these changes?
For example - https://imgur.com/WkL8RaP . Also I don't see the need to announce it, just say it in the update note and then players will spread the word and most will be aware of the restrictions.
3. What happens when players inevitably find out their settings are automatically changed, whether they realize or not, and crash?
If I'm not mistaken, there are now some restrictions for using the settings of some parameters. So if the console is used - it is a message in the console, if the standard setting in the game - the parameter is set to the maximum or minimum value, depending on what the player did, increased or decreased the parameter.
I don't see a meaningful connection to the rain. The physics change isn't that significant, and if restrictions for intensity are added - that difference is almost invisible, but now yes, it makes a big enough difference, so that's still a positive side to this idea.
and how that could be applied in a way which all players consent to.
It is enough to explain the reasons for doing so. And, as a rule, most players who use limit values do so for unsafe driving, so for normal players who follow the rules and stick to the simulation it is unlikely to have any negative effect.
Why not think bigger?
I think the text below is not part of the original discussion and has a different meaning, so I'd say it's not relevant to the current thread.