What is the religious ruling on thinning very sparse eyebrows by plucking (namṣ), henna, or dyeing, when family and friends request it?

Plucking is forbidden. As for dyeing hair with henna and similar substances, it is permissible as long as black color is avoided. There is no harm in transplanting hair in the eyebrows if they have a disfiguring and noticeable deformity, based on the Prophet's (peace be upon him) command to Arfajah ibn Sa'd to have a nose made of gold.