ATM git-annex init on a new clone would try to sense the remote regarding git-annex support and would try to fetch its /config. If that fails it would announce that remote ignored for annex.

...
Switched to branch 'adjusted/master(unlocked)'
download failed: Not Found

  Remote origin not usable by git-annex; setting annex-ignore

and that download failed, thanks to --debug output comes from

[2018-09-28 12:52:12.541757374] Request {
  host                 = "github.com"
  port                 = 443
  secure               = True
  requestHeaders       = [("Range","bytes=0-"),("Accept-Encoding","identity")]
  path                 = "/psychoinformatics-de/studyforrest-data-structural/config"
  queryString          = ""
  method               = "GET"
  proxy                = Nothing
  rawBody              = False
  redirectCount        = 10
  responseTimeout      = ResponseTimeoutDefault
  requestVersion       = HTTP/1.1
}

download failed: Not Found

  Remote origin not usable by git-annex; setting annex-ignore

IMHO that "download failed: " message output should be provided only in --debug mode. Otherwise, if exposed to naive user it is an uninformative alarm, which (if he/she knows) is clarified by the follow up message ("Remote origin not usable ...")

ref: https://github.com/datalad/datalad/pull/2881#issue-218977359

done --Joey