I will break this down into two parts - personal experience and generic effects.
Even though I did not DM much campaigns, in those that I did I shared my notes and ideas for the campaign after it ended and answered questions regarding the player's actions after sessions in case the answer did not affect the story any further.
I think this approach is heavily dependent on the players themselves. In our group it was something they enjoyed. It was just another piece of our session-wrap-up, where we discuss with DM what we liked or disliked about the session.
Sharing these ideas is not only group dependent, as mentioned above, but also story dependent. To make note sharing have some effect, the story must have branches that were not explored, ideas that were not used, monsters that were not defeated an so on.
If you have an adventure, which is less open world and more just dungeon crawl / one-already-decided-storyline, there is nothing much to share with players that they would not encounter in the campaign itself.
However in case your campaign offers variety of options to do, being prepared for most of them and then using some according to player's actions leaves you with some unused material. There you have to decide if you want to reuse it in the future.
Players might want to know how else could they get in the castle but if you give them the list of possible ways, next time you put castle in your story, they will already know your last options and will probably just take one of them -> you will have to either prepare different ones or deal with it.
I will break possible positive and negative effects down as I see them.
- Players will be motivated to expore more so they won't miss so much in next campaign
- It will answer some questions about the story that were unanswered because of the game development (why did that guy give us in to the guards? -because you killed his father in the forest in the very beginning)
- Players will get new ideas and perhaps try different approach next campaign depending on how their actions affected the current one (for example if they see that you had planned whole section for sneaky approach, they will be more motivated to try doing that next time instead of kicking down doors and killing everyone in the sounds of battle horn from their dwarf bard...)
- You as a DM can share and talk about things that you could not during the campaign (guys, I have no idea how you were able to miss this, I gave you this hint like 4 times -now that you mention this, he DID look a bit suspicious)
- Probably many more
- You lose content (whatever you share, you can't reuse in the near future with the same group and achieve the effect of surprise)
- You can't easily create mysterious characters appearing in more adventures as if you reveal that the blacksmith was important and in the next campaign, they meet him again, they will probably remember...
- Some players might not like it because they like the thrill of surprise, of not knowing the "behind the scenes" and even if they retain from reading your notes themselves, if the rest of the group knows these things, it could potentially cause troubles and tension later (oh this guy looks similar to the secret agent that DM had in the last campaign we did not discover, we better be more careful around this guy, just to be sure)
- New players could be affected. Just like in the previous case, new players that join your group will be in a spot where they would miss some meta information and might feel handicapped by it (oh how was I supposed to know that this trap deals so much damage -well we did avoid this statue in our last encounter and DM had it as a special trap, seems he reused it here...) or alternatively (watch out for this statue, last adventure, these statues contained deadly traps so check them in advance... -great now I missed on a deadly surprise)
- Probably many more
To sum it up, in general I think it boils down to the players themselves. If you have group that is happy with knowing the ropes and tricks that pull the story and you don't have issues with sharing your notes, I think it is a good way to give players a bit more fun - maybe meta-talk the game after the campaign ends, discover the routes that were cut off and so on...
However, when it comes to new players joining your group, this is something they should be familiar in advance so that you avoid possible misunderstandings later.
Finally, keep in mind that sharing your notes puts much more pressure on you, as a DM, to come up with fresh and original content for the following adventure.