EDIT: Below was in answer to the original High Sierra question, but it appears that the original question has changed when the hardware issue came to light. I am leaving the answer below posted in case others find it useful when coming across this question. The hardware should either be fixed or a third party WiFi USB dongle that supports 5GHz 802.11n or 802.11ac can be used to re-enable AirDrop.
Apple has not removed Airdrop from macOS 10.13 "High Sierra" but if one doesn't see it in the Finder Sidebar, try restoring it.
This article from Ars Technica review High Sierra and computable Macs.
From the article:
For a handful of macOS features, mostly related to graphics and wireless communications, you’ll need a system that’s slightly newer
than the minimum baseline. For “Continuity” features like AirDrop and
Handoff, you’ll need a Mac with both Bluetooth 4.0 and a 5GHz 802.11n
or 802.11ac Wi-Fi adapter:
MacBook (Early 2015 and later)
iMac (Late 2012 and later)
MacBook Air (Mid 2012 and later)
MacBook Pro (Mid 2012 and later)
Mac Mini (Late 2012 and later)
Mac Pro (Late 2013)