Whyever would we not accept questions about Youtube downloaders? The topic of this site is software recommendations, not software-that-doesn't-download-from-Youtube recommendations.
On the legality of using Youtube downloaders
In my jurisdiction, it is legal to make copies of any content covered by copyright (a book, a CD, a website, …) for personal use; anything else requires the permission of the publisher. As far as I understand — but I am not a lawyer — this means that I am allowed to download Youtube videos. This applies only if I am an anonymous visitor: if I use a Youtube account, then a contract would bind Youtube and me.
DADVSI law (the French equivalent of DMCA) puts an additional restriction that if Google puts technical restrictions on copying Youtube videos, then I am not allowed to work around them. Again, this is my understanding as a non-lawyer. If a piece of software can download Youtube videos, then presumably it is not hampered by these technical restrictions and therefore my copying is allowed. I believe that most EU countries has a similar law (national implementations of EUCD), and there is a similar provision is the US DMCA.
Now it is possible that the downloader software is, in fact, working around a copying restriction. I would need to be both a technical and a legal expert to determine that; this is well beyond me. As a general principle, the law assumes that something is legal unless it has been proved not to be. As a private citizen, I assume that something is legal unless I have a serious presumption that it isn't. Being able to download videos from Youtube does not constitute a serious presumption of illegality, seeing that most web browsers can do it.
On the legality of discussing Youtube downloaders
Stack Exchange operates under US law. US law has very strong free speech provisions, so if you think that discussing something is illegal, you need to have very strong arguments. DMCA is a reason why something might be illegal to discuss: it makes it illegal to discuss acts that are themselves illegal under DMCA. (Have I already mentioned that I am not a lawyer and this is my layman's interpretation?) DMCA has a very specific procedure to censor content that is illegal under DMCA: the copyright owner or a legally appointed representative must contact the content provider. The procedure for doing this is listed at the bottom of the Stack Exchange terms of service. The site's community, has nothing to do with this. In particular, moderators, who are community members, are not agents of Stack Exchange and are thus not empowered to enforce copyright counterclaims.
Nothing I can see in the Stack Exchange terms of service would make discussions about downloading Youtube videos prohibited content. Content that “infringes any intellectual property right of another” is prohibited, but as explained above, to my knowledge, discussions of Youtube downloaders do not infringe any intellectual property right of Google or another party.
Ethics are a personal matter. If your ethics prohibit you from participating in discussions related to Youtube videos, please ignore the youtube tag, and you should be spared.
We will not block a topic from discussion unless there is a broad consensus that it should be blocked. The Stack Exchange terms of service prohibit content that “ is libelous, defamatory, abusive, threatening, harassing, hateful, offensive”; I do not see how Youtube downloaders intrinsically fit any of these qualifiers.
If you want to censor discussions of Youtube downloaders or declare them off-topic, you'll have to come up with better arguments that “someone on the Internet says it may or may not be legal”.