Here in the Community, we don't have the specific timeline as to when a feature will be added.
In the meantime, you have the option to access your account in a new browser application. From there, you can manually track your mileage.
For your reference, here's an article you can to learn more about manually track your mileage: How to Track Mileage?
In addition, you can also enter your employee's timesheet in the web version of QuickBooks Online.
That being said, you can check out this article for your guide in recording your employee timesheet: How to Enter a Single Time Activity Timesheet?
Lastly, you can also visit the QuickBooks Online Blog. This way, you'll be updated to the latest features being rolled out in QBO.
I'll be sure to help you out here in the Community if you have any other concerns. Just leave it in the comment section down below.