Still looking for what it does exactly but
https://wow.gamepedia.com/Patch_8.3.0/API_changes
mentions a few things that might be worth checking out:

    C_QuestLog.GetActiveThreatMaps
    C_QuestLog.HasActiveThreats
    C_QuestLog.IsThreatQuest
    C_TaskQuest.GetThreatQuests

Dump: value=C_TaskQuest.GetThreatQuests()
[1]={
[1]=55350,
[2]=56064,
[3]=56308,
[4]=57008,
[5]=57157,
[6]=57728
}

The next release of Grail records the currently available threat quests using this API.