Short answer
Yes. Shaving slits into the eyebrows is not allowed in the Hanafi school, because it removes eyebrow hair for style, which falls under namas (plucking or thinning the eyebrows) that the Prophet ﷺ cursed. It also copies a street fashion. Men may not shape or thread their eyebrows, but they may trim very long stray hairs and remove hair between joined brows.
Namas, removing eyebrow hair to thin or shape the brows for beauty, is haram. Trimming hairs that are unusually long or growing out of place is permitted, and removing the hair between joined eyebrows is allowed by many Hanafi muftis, as it is not part of the brow itself.
The four schools agree on this.
Explanation
The hadith that curses tattooing also curses al-namisa and al-mutanammisa: the woman who plucks eyebrow hair and the woman who has it done. Ibn Mas'ud explained that such people change the creation of Allah for beauty. Shaving a slit, line or gap into the eyebrow is removing eyebrow hair to change its look, so it falls under the same prohibition. The four schools agree on the basic ruling of namas.
For men there is a second problem. Shaping, plucking or threading eyebrows is a women's beauty practice, and the Prophet cursed men who imitate women. Slits are also a trend associated with gangs and street culture in many places, which a Muslim does not need to copy.
What is allowed. You may trim a few hairs that are unusually long or that grow down into the eye. Many Hanafi muftis allow removing the hair between joined eyebrows (a monobrow), since that hair is not the brow itself. Treating a scar, a burn or a skin condition is a medical matter, not namas.
If you already have slits. Let them grow back. No other action is needed, and your prayers are valid. Questions about women shaping their eyebrows are covered in the women section.
The evidence
Quran, Surah An-Nisaa 4:119
وَلَأُضِلَّنَّهُمْ وَلَأُمَنِّيَنَّهُمْ وَلَءَامُرَنَّهُمْ فَلَيُبَتِّكُنَّ ءَاذَانَ ٱلْأَنْعَٰمِ وَلَءَامُرَنَّهُمْ فَلَيُغَيِّرُنَّ خَلْقَ ٱللَّهِ ۚ وَمَن يَتَّخِذِ ٱلشَّيْطَٰنَ وَلِيًّۭا مِّن دُونِ ٱللَّهِ فَقَدْ خَسِرَ خُسْرَانًۭا مُّبِينًۭا ﴿١١٩﴾
And I will mislead them, and I will arouse in them [sinful] desires, and I will command them so they will slit the ears of cattle, and I will command them so they will change the creation of Allah." And whoever takes Satan as an ally instead of Allah has certainly sustained a clear loss.
Changing Allah's creation for fashion is from Satan's call.
Sahih Muslim 2125
Sahih... لعن الله الواشمات والمستوشمات والنامصات والمتنمصات ...
"Allah has cursed the women who tattoo and those who have it done, and those who pluck the eyebrows and those who have it done."
The prohibition of namas.
Sahih al-Bukhari 5885
Sahih... لعن رسول الله صلى الله عليه وسلم المتشبهين من الرجال بالنساء ...
"The Messenger of Allah cursed the men who imitate women and the women who imitate men."
Men shaping their eyebrows imitate women.
Where the ruling is recorded
- al-Durr al-Mukhtar with Radd al-Muhtar, Kitab al-Hazr wa al-Ibaha. Namas is forbidden, with the exceptions for abnormal hair.
- al-Fatawa al-Hindiyya, Kitab al-Karahiyya, chapter on adornment, rings and jewellery. Removing facial hair and eyebrow hair.
This answer is pending review by the scholars on our team. Quran translation by Sahih International. Hadith Arabic from the standard editions of the six books and the Muwatta, with gradings as recorded there; the English is our own rendering. Spotted a mistake? Tell us.