I don't know the passcode for 16 year old DSDs phone. I don't think my DH does either. Not sure how much we'd gain from going through her phone at this point. The last time we did, she was 14 and that went so badly (we may as well have helped ourselves to her diary), that we had to figure out a better approach. We've been able to get a more productive conversation out of her by just talking with her (which sometimes involves sending her WhatsApp messages despite being in the same house).
That said, I do still have screen time monitoring and limits in place through the Apple family settings. She has to get permission from me to add an app, and when I want to, I can see how much time she's spending on which apps and limit the ones she can use. I still imposed Downtime this year to get her off her phone by midnight.
TBH, I don't remember my parents knowing my AIM or MSN Messenger passwords. They definitely didn't know the passwords for all of the different message boards and email lists I was part of back in the day. They had no access to my email once my friends taught me how to set up my own Hotmail account at age 14 (thank you summer camp).
They could have peered over my shoulder while I accessed these things on the family PC in the living room, but they rarely bothered. We had more and better conversations when they made an effort to watch TV shows I liked with me, or listen to a band I liked.