Yahoo answers is/was a US-based site. (It could be accessed though a .co.uk web address but visitors still ended up on the main US site). Like the vast majority of general Q & A forums, it suffered from a lack of proper moderation and was thus full of posts from trolls and spammers.
Answerbag.com is/was a similar, US-based, site. It got so full of spam and troll posts that its owners closed it down. However it was then bought by a different company, who promised that things would be a lot different. They weren't.
Several other Q & A websites have closed down because of similar problems.
Before any Q & A sites ever existed, in the very earliest days of the internet, there was Usenet (which was separate to the World Wise Web), where anyone could create a forum for a topic of their choice. For example, there was a forum for owners of English Setters and another one for discussing Radio 4. However the system was entirely unmoderated, eventually meaning that for every one post on a forum about, say, traction engines, there were over a thousand posts advertising sex sites, etc. So Usenet fell into disuse.
Other sites have fallen by the wayside because they weren't adequately moderated. (e.g. Sagazone, which had the funding of the massive Saga group behind it, was meant to be a discussion forum for people over 50 but it ended up full of troll posts from right-wing extremists).
At the moment there are very few 'general' Q & A sites available anywhere in the world. Quora seems to run well, although it's not as user-friendly as The Answerbank is. StackExchange is hard to navigate, with its most active sections being those devoted to computer technology (while others are hardly used) but it appears to be well run.
There are also plenty of other well-run forums dedicated to specific subjects (such as web design) but good general Q & A websites are hard to find!