I just came across a Reddit comment referencing the Human Planet documentary about sky burials, in which there was an implication by the narrator that the rogyapa (body-breaker) performing the ritual must be a non-Buddhist (around 2:40). The Reddit comment states that breaking the body apart is a form of defilement, thus not appropriate for monks. Is this true, and is there any basis in Buddhist literature supporting such view?
Quoting from the Pali texts (not Tibetan literature)...
The first thing I notice is in the Mahāparinibbānasutta (DN 16):
Then Anuruddha said to Ānanda, “Go, Ānanda, into Kusinārā and inform the Mallas: ‘Vāseṭṭhas, the Buddha has become fully extinguished. Please come at your convenience.’”
Or I've seen this translation:
At that time the Mallas of Kusinara had gathered in the council hall to consider that very matter. And the Venerable Ananda approached them and announced: "The Blessed One, Vasetthas, has passed away. Do now as seems fitting to you."
In summary it's the responsibility of the local lay society.
Looking into the Vinaya I find this:
Unlike some of the other early Vinayas, the Pali Vinaya contains no rules on how to conduct the funeral of a dead bhikkhu or novice. Writers have speculated as to why this is so, but the speculation tends to say more about the writers than about the Vinaya. The practical upshot is that the Community (or the bhikkhu’s friends, relatives, etc.) may dispose of his body as they see fit in line with local custom and law. DN 16 states that an arahant, after death, deserves to have a stūpa built over his/her remains, but the Vinaya contains no rule to enforce this.
One issue that arises at present is the custom of willing one’s body to medical science. Because there is no rule that the bhikkhu’s body (as opposed to his belongings) belongs to the Community, if he has willed his body in this way his wishes may be honored.
Another issue arising at present is the cost of a funeral. In the Buddha’s time, funerals could cost nothing. The body would either be cremated, in which case wood was easy to find in the ubiquitous forest, or the body would be exposed in a charnel ground, which involved little effort and no expense. At present, with the high cost of funerals, the tradition in Thailand is a useful adaptation of the Vinaya’s rules. There, if no one else volunteers to sponsor a dead bhikkhu’s funeral, the Community itself is the sponsor, and the funds for the funeral come first from his belongings. Only if any of his light articles and requisites remain after the funeral are they divided among the Community’s members.
That doesn't say it's "considered unclean" -- but does continue the theme of its being a responsibility for lay-folk -- and perhaps in that way, "not appropriate for monks".
Also I do gather that in Tibetan society, a lot of the "manual labour" tasks were done by lay-people -- I think that's generally true of other Buddhist societies too, though apparently monks may do some forms of physical work (e.g. building or maintaining a monastery).
I get the impression that there were no "body-breakers" in the society associated with the Pali texts -- that corpses were taken intact to "charnel grounds" (Sivathika) and left there unbroken to decay.
Incidentally I'd normally associate the word "unclean" as referring to a Western understanding of the Indian caste system -- see Untouchability -- whose jobs tended to be literally/physically polluting e.g. emptying latrines.
In the past, they were believed to be so impure that caste Hindus considered their presence to be polluting. The "impure status" was related to their historic hereditary occupations that caste Hindus considered to be "polluting" or debased, such as working with leather, disposing of dead animals, manual scavenging, or sanitation work.
I don't know if there was an equivalent practice in Tibetan society.
I guess that dismembering corpses might be unclean or polluting -- dangerous for medical reasons -- so if it were considered ritually or professionally unclean that might be part of the reason (though this is not exactly what you were asking for).