From context it is very clear to not divulge or public expose that Vidya. If the practice was forbidden then the furthermore text shall tell us to avoid or strictly forbid: to study (read) ,to meditate 3 times (three divisions of the day [dawn , noon , and sunset] ) on this Devi, which is not the case:
"paThanIyaM mahAdevi trisandhyaM dhyAnatatparaiH"
So the forbidden is only the expose to others (public) the Vidya and this stereotype is not limited to this or any other MahaVidya. Having in mind those texts actually (historically) have take life in Kali Yuga, this is the correct interpretation of such text instance.
Keep in mind Na Prakashyam is indeed to not divulge secret to others, and appears in lot of texts (e.g. "Kalishatanamastotra" etc) . For exact translation of prakAshya , while "na" have the negation sense.
David Frawley Tantric Yoga and the Wisdom Goddesses: Spiritual Secrets of Ayurveda, quotes:
“Perceived as the Void, as the dissolved form of consciousness, when all beings are dissolved in sleep in the supreme Brahman, having swallowed the entire universe, the seer-poets call her the most glorious and the eldest, Dhumavati. She exists in the forms of sleep, lack of memory, illusion, and dullness in the creatures immersed in the illusion of the world, but among the yogis she becomes the power that destroys all thoughts, indeed Samadhi itself” — Ganapati Muni, Uma Sahasram 38. 13 -14
Ganapati Muni's full text can be find on Uma Sahasram
A reference I found also:
David R. Kinsley Tantric Visions of the Divine Feminine: The Ten Mahāvidyās (Motilal Banarsidass Publ., 1998, P.183-184 ):
“An inkling of Dhumavati's positive aspect is suggested in a comment made to me by a priest serving the Dasamahavidya panda (a temporary shelter for worship) on S. N. Banerjea Street during Kali Puja in Calcutta. After telling me that she is a vision of old age and decay—that she is nearly blind and has loose, wrinkled skin, sagging breasts, and no teeth—and that furthermore she looks fierce, he said that inside she is tenderhearted. The priest at the Dhumavati temple in Varanasi, Panalal Gosvami, after telling me about all the inauspicious aspects of the goddess, and emphasizing that she should not be approached by happily married men like me, said that she gives "anything the devotee wants," which he said was unusual among deities. He also said that her worship instills a feeling of wanting to be alone and a distaste for worldly things. In this vein, he said, her worship is appropriate for world renouncers. He also said that Dhumavati is partial to unmarried people and to those who have been widowed. He insisted that only unmarried people could withstand her great power and successfully spend a night alone in her temple. For a married person to do this, he said, would result in death.”
You can also see this response other fierce forms of Devi
There are some cases in which not particular this Vidya is forbidden for some categories.
We understand as well some practices were forbidden, special those which imply destructive actions to others, because in texts describing this Vidya, as Kaularahasya for example, we found that Dhumavati kavaca is used for punishment of enemy (asti guhyatamaṃ loke kavacaṃ śatru nigrahaṃ ), death of enemy (śatruṇāṃ māraṇā ), killing of enemy (śatru mardini) and so on.
Hope this helps.