I think it depends on what it is. Dp and I have strict rules because we work together. We have different roles and mine is effectively more senior. That means that I know things that I cannot tell him and he is fine with that.
On the flip side he knows there are certain things I have to respond to in an official way when it might not the best option , he chooses what to share. However we have done this a good while and have both been in an industry that requires confidentiality for over twenty years.
We are crystal clear where the line is that we would need to let the other one know professionally.
It confuses our de facto boss , if dp does something wrong , it took a long way for our boss to realise I would deal with it in the same way I would with anyone else and not pre warn DP. However now it's common knowledge that we are not in a relationship at work...that would not be possible if people thought we discussed everything.
We do have a rule though , if it is something that id genuinely distressing us and we need to talk to each other as partners for comfort we have a code word (which is ridiculous as it is falafel...but that's because it's a word that never ever comes up in the house so it's clearly identifiable)
If one of us says that we can talk without risk of it ever being referred to or repeated outside of us two.
It also depends on what it is , risk to someone else is crossing the line and neither would keep quiet.
I find it weird that some tell each other everything. I don't actively keep things other than at work from DP, but frankly he doesn't care if my friend and her partner are arguing, why would I need to repeat it?
It feels a bit try hard to HAVE to tell your partner everything , a bit like the relationship you have at 17 that you are desperate to be grown up in?