In our marriage, it was never discussed, but we are not secretive about our phones. I will happily ask my wife to go into my emails and find some registration code, check something on my whatsapp or look up something on my instagram, log into my work chats while I am driving, for example. She is also logged into all the above on her laptop that we use for amazon orders, and it is home with me when she is away to work. There is nothing to see, and it never even crosses our minds to check on the other person, either. Our phones are left in the same room without a second thought when we go and take a shower or go outside gardening, etc.
I can't imagine being with someone who is so insecure about our relationship that it makes her not trust me, nor would I be able to maintain a relationship with someone I can't trust.
There are so many things in life that drain our energy and attention, it's just such a waste to put it into these. And if you are with someone who needs to be checked, that relationship is dead, to begin with. I could maybe wrap my head around someone being cheated on before, being sensitive about it as a result, but that would take a lot of clear communication to fix what's broken there. At the same time, I wouldn't be comfortable to be treated like a potential liar and a cheat inherently, just because some other man has cheated in the past - It is not my fault, it's not fair to be punished for it.