"In many cases additional letters are used, some of which are adaptations of standard Cyrillic letters, while others are taken from the Greek or Latin alphabets."
Except in those cases, the letters are encoded in Unicode in the Extended Cyrillic range, and in no case is the LATIN SMALL LETTER H used.
Yes, this is an IDN in the technical sense, but it is not an IDN in the sense that it is an International Domain Name.
What it is is an immature phishing type of mixed script domain, that has been giving legitimate International domains a bad rep, as we all know from the news. Partly because of this, language groups that really need non-latin domains have been given the short end of the stick, because the actions of a few phishers coming from the Latin-1 regions mostly that have made news. This has already led to unnecessary restrictions on their use in major browsers and has delayed implementation.
Don't try to cover up the issue by posting things about the history of Cyrillic. That has no relevance to the discussion.
It isn't for no reason major registrars today prohibit mixed script domains.