I am running Slackware ARM 14.2 on a RP 3B+. I saw that some people had issues with wifi. My wlan0 device is being recognized, but it shows no results. "iwlist wlan0 scan" comes back with
"wlan0 No scan results. "

I know there are plenty of strong wifi signals nearby.

I also edited the /etc/wpa_supplicant.conf and added "country=US", but that does not seem to make any difference. The exact same setup works fine on the RP 3B (I have three of them running, and they are all connected via wifi). This issue seems specific to the 3B+. Any thoughts?

Give the official kernel a try, if you're not already using it.
Follow the second half of this post, starting from "Follow the manual installation method, focus":
https://www.linuxquestions.org/quest...4/#post5862696
And use the latest Raspberry provided image to extract the kernel&stuff:
http://director.downloads.raspberryp...retch-lite.zip

P.S.
https://www.raspberrypi.org/forums/v...?f=28&t=209409