The roadmap added a mobile device icon to the PC and tablet symbols with the "Extensions" listing on the Windows for Business roadmap section (via Plaffo) titled "In Development.".Microsoft previously hinted that it had no plans to offer extension support for the Edge browser in Windows 10 Mobile.
However, keep in mind that this feature's appearance on a roadmap does not guarantee that it will eventually be released. Indeed, Microsoft's roadmap offers this caveat:
The information on this page is for informational purposes only, is subject to change at any time without notice, and may not be applicable to customers in all geographies.
In any case, we don't expect that it will be added to Windows 10 Mobile as part of the big Windows 10 Anniversary Update.