Using Classic Outlook on Windows for personal email, calendar, and contact management
I have Outlook 365 Version 2403 (Build 17425.20146) and I am facing the same issue.
Around a month ago I had this button, then it was gone for a week or two then came back for a day and disappeared for good ;)
It is a very helpful button and I want it back.
Anticipating future advice, to be honest, I don't care about the functionality of the automatic focus plan in Viva Insights. It doesn't work the way I need to book my Focus Time. In my case, it works ad hoc, so I need a button which can quickly help me to schedule Focus time on-the-go.