I think it's something of a matter of personal style, or if writing for publication, the style guide of the intended publication. The name of the coding method is "UTF-8" (cf. http://en.wikipedia.org/wiki/UTF-8), so the hyphen between the character "F" and the character "8" is a part of the name. But looking at the first link you provide, the hyphen is not required from case 1 because the two parts, "UTF-8" and "encoded" do not have a combined meaning that differs to any significant degree from the phrase "encoded with UTF-8". It's also not required from case 2, because as pronounced, "8" ends in a consonant, and "encoded" begins with a vowel.
On the other hand, according to your link to question 889, one could argue that there should be a hyphen, but according to that site, only in the exceptional case that the phrase precedes the noun it modifies, as in "UTF-8-encoded document". But it's not clear to me that this exception applies in the absence of the following noun, for example in the construct "The document is UTF-8 encoded." And since this last statement is functionally equivalent to "The document is encoded in UTF-8.", I don't think that the hyphen is necessary. But I don't think a hard and fast rule applies here, and I can conceive of cases where I might see the construct with a hyphen between the "f" and the "8", and between the "8" and the following word might be useful.