It was removed in v6. In my opinion, it's a huge setback and a huge SECURITY issue. I was horrified when I went to my system tray to open Enpass and I was already logged in!
I mean, you can't remove such a key feature just like that, especially when people have had this setting configured for ages and are used to the application locking itself after being inactive for X amount of time. Many people might not even find out about this for some time, leaving their account data potentially exposed to anyone else who -even by chance- opens the application.
Enpass Team, please do right by your users and fix this security issue.