Anyone who insults the Prophet ﷺ clearly and unequivocally is kafir without doubt. Being Barelvi or Deobandi has no impact on this. The rule is the same for all.
وَاعْتَصِمُوا بِحَبْلِ اللَّهِ جَمِيعًا وَلَا تَفَرَّقُوا
Hold firmly to the rope of Allah together, and do not be divided (into factions). (Surah Ali Imran, 103).
The way this question has been worded is concerning, it gives the impression that there is a different set of rules for certain groups. Unfortunately, it seems that this is not merely incidental in the question, rather a practice many people have. Many Muslims in our time have divided into recently established sects, rather than following the classical and agreed upon scholars in Islam, they place authority with these recent or contemporary scholars. This is not only a problem in itself, but also leads to many other problems.
One such problem has been alluded to in the question above. Many of these people attack other sects and point out issues they have, sometimes this is substantiated and other times it is not. However, they will justify or ignore similar things found within their own groups. This approach is not only unjust but also demonstrates that this person’s allegiance is to the group or sect not to Islam. If the allegiance was truly to Islam then he would oppose corrupt teachings wherever he finds them.
Another problem with many of the sects we have in the Ummah today, is they sometimes misrepresent what others have said in order to attack them. This approach is obviously unacceptable and often leads to them ignoring mistakes made by members of their own sect and highlighting the mistakes of others. This is just one reason why sects are dangerous.
The verse of the Holy Quran above clearly prohibits us from dividing into factions, yet many Muslims misinterpret or misunderstand it and continue to divide.
وَلا تَفَرَّقُوا: أَمَرَهُمْ بِالْجَمَاعَةِ وَنَهَاهُمْ عَنِ التَّفْرِقَةِ، وَقَدْ وَرَدَتِ الْأَحَادِيثُ الْمُتَعَدِّدَةُ بِالنَّهْيِ عَنِ التَّفَرُّقِ، وَالْأَمْرِ بِالِاجْتِمَاعِ وَالِائْتِلَافِ (تقسير القرآن العظيم)
And do not be divided (into factions): He (Most High) has commanded them to unity and prohibited them from division. Indeed numerous Hadith have been narrated regarding the prohibition of dividing (into factions) and the command to remain united and in harmony. (Imam Abu al-Fidaa Ibn Katheer, Tafseer al-Quran al-Adheem).
I cannot comment on things found in the books of the Barelvis or Deobandis because I do not read their books, rather I tend to read the books of classical scholars. However, I have clearly answered the question regarding insulting the Holy Prophet (peace be upon him) above. Whether this is applicable to a particular statement found in this or that book of the Barelvis or Deobandis is a question for someone familiar with their books.
I would advise people to stay away from all sects and follow the mainstream of Ahl Sunnah and take from classical Scholars that the Ummah has agreed upon.
Below are links to two videos that will further clarify what has been mentioned here.
And Allah (Most High) Knows Best.
– Answered by Shaykh Noorud-deen (29.03.2021)
See also (video):