As you may already know, Microsoft has released Windows 10 build 15058 recently. This is the latest build as of this writing which represents the upcoming Windows 10 Creators Update. It changes the policy on how Windows Update will work when your connection is metered.
Once you open the Settings app and dive into Settings - Update & security - Windows Update, a new text banner informs that some updates will still be downloaded over a metered connection:
A lot of users use the Metered connection feature to prevent Windows 10 from downloading and installing updates. It looks like guys from Redmond are not happy with this situation. Windows 10 Creators Update will change this.
The updated text has a portion that says:
In that case, we'll automatically download only those updates required to keep Windows running smoothly.
It is not clear which updates will be installed exactly over a metered connection. But if you are on a limited data plan, prepare to get a higher internet/data bill once Windows 10 Creators Update arrives on your device.