Is it considered namas to thin the eyebrows, and does it entail the curse mentioned in the noble Hadith: "May Allah curse the namisah and the mutanamisah"?

The hadith (definition) mentioned in the question is authentic, beyond any doubt. It was narrated by Al-Bukhari, Muslim, and others with various wordings. It was also reported in Sahih (definition) Muslim on the authority of Ibn Mas'ud that the Prophet, peace and blessings be upon him, cursed tattooists and those who get tattooed, those who pluck eyebrows and those who have them plucked, and those who create gaps between their teeth for beauty, altering Allah's creation. As for some people's non-adherence to the Sharia (definition), this is not an argument against the authenticity of the hadith.