Microsoft previously promised extensions support would arrive later this year, but now WinBeta heard there will be no further feature updates for Edge after next month's Threshold 2 update.
Based on the software giant's current planning, it seems extensions support is delayed quite a bit and won't arrive until the Redstone update in Summer 2016:
Insiders will likely get to mess with extensions before the summer however, as the Insider Program will likely begin receiving new Redstone builds way before RTM is released to the public. Microsoft will want developers and insiders to test (and develop) extensions within Edge, so when the update launches consumers will already have a number of extensions available for the browser. For those who don’t know, Microsoft is working on a bridge to allow extensions developed for Chrome to be easily ported to Edge with little to no code changes, much like the Project Astoria bridge.