Looks like Microsoft has a new release of the Phone app for Windows 10 Mobile and Windows 10. Version 2.12.2002.0 is now available, which is a rather big version change from the previous 1.10.23004.0.
However, the only discernable change we noticed and cannot confirm is when dialing voicemail on our dual-SIM phone we are no longer prompted to choose which SIM to make the call. However, this could be just an isolated observation as you could always choose the SIM to make calls from in the upper right-hand corner.
If you notice any other changes to the app on Windows 10 Mobile or for PC let us know in comments.
Updated: As noted in comments we can verify that Call Duration is back under Details (long press on a call in the history).
To check for the update just head to your Store and use the Check for updates function.