All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger.
The Khawaarij did not become extinct after the Battle of Nahrawan; they exist today and will continue to exist until Ad-Dajjaal (Antichrist) emerges.
The Prophet, sallallahu ‘alayhi wa sallam, said, "...Whenever a group of them appears, they should be cut off (i.e. killed)." Ibn ‘Umar, may Allah be pleased with them, said, "I heard the Messenger of Allah, sallallahu ‘alayhi wa sallam, say, 'Whenever a group of them appears, they should be killed' - (he said it) more than twenty times - 'until Ad-Dajjaal emerges among them." [Ibn Maajah - Al-Albaani and Al-Arnaa’oot graded it hasan (good)] Al-Booseeri said in his book Az-Zawaa'id that the chain of narration of this hadeeth is saheeh (sound).
Another version of the hadeeth was cited by An-Nasaa'i and Ahmad with a weak chain of narration. It reads, "...They will not cease to appear until the last of them comes with Al-Maseeh Ad-Dajjaal (Antichrist)."
It is important to note that many of those accused of being Khawaarij in this age are not fit for that description. They are not really Khawaarij. The criterion in labeling someone as a Kharijite is his adherence to the false beliefs of the early Khawaarij. Whoever conforms to their beliefs is one of them. Ibn Hazm wrote, "Whoever agrees with the Khawaarij in their denial of arbitration, the Takfeer (declaring a person or a group to be disbelievers) of those who commit grave sins, approval of rebellion against the Imaams of oppression, that those who commit grave sins will abide in Hellfire eternally, and that the position of Imaam may be held by other than the members of Quraysh tribe, then he is a Kharijite ..." [Al-Fisal fil-Milal wal-Ahwaa' wan-Nihal]
We have previously discussed the emergence of the Khawaarij and their fundamental tenets in fatwas 84776 and 91562.
Allah knows best.