The content, in general, is quite thin
that's not how thin content works. When google penalizes sites for "thin content" it means that those sites have little content (ie. very few words) overall, not in the comments / Q&A sections.
So, if your site only has - as an example - 3 articles of 1000 words each, this is what google considers thin content and it's why your site might not rank and it has nothing to do with the questions / comments that your site gets, which actually do ADD to the content tally of your site, so you want to get comments / UGC whenever possible...
Personally, I don't know the limit for what google considers thin content, but I think that if you have less than 10-15 pages and / or 10-15k words then your site could be marked as thin content. Just aim to have at least 30 content pages with each page having at least 600-700 words (regardless of how much UGC you have) and it should be enough not to have to worry about thin content anymore.
and there are some duplicate questions
it doesn't matter
However, I still find them somewhat useful.
so keep them! If google want to penalize you they'll do it even if you have the best site in the Universe
Nevertheless, most of these Q&A posts receive little to no traffic
pages that get no traffic might not rank, I have heard of this before so it's probably true. But if those questions are on a page that does get traffic (although, from what I'm understanding, your questions are on a separate page, is that true?) you're good and you should keep the page.
After Google's algorithm update in October, they were targeted as thin content.
how do you know? I just checked google's official updates release page and I'm not seeing anything specific about thin content. Only the same general guidelines...
May I ask for a link to where you've read this?
Should I use 'nofollow' for all these Q&A posts, or if not, how should I handle them?
just don't stress about them, let them exist if you feel they bring value to your visitors and don't worry about doing anything with them. Nofollow is usually used for external links (ie. you link out to Wikipedia or to WebMD for example, that's when you use nofollow), but when linking to anything on your site - although it can be done when you don't want to boost the authority of pages that you have no intention of ranking - you don't use nofollow. Nofollow simply tells google that you don't trust the website / page you are linking to but if you nofollow your own content then this tells google that you don't trust your own content, and most of the time you don't want this
