Yes, but only when the information is not widely known or reasonably obvious to current-day users, and the explanation is not given by a question or answer on a StackExchange site.
One heuristic is whether or not we have a tag for the thing in question. If we have a tag for it, it's probably (but not always) well known enough that a link to an explanation ...
This is a tricky one.
I think the easiest solution is to go with our usual link policy, and require the question to be fully understandable without the link. This requires providing some sort of text-based description that can be used to easily find the content described by the link – for example, describing the basic attributes of the advert like ...
In general each and every question should be self contained in text (*1). Every other media is usually neither searchable nor always displayable. Also, describing picture/video/recording content in a textual manner does allow to put emphasis on components relevant on the question.
If other media gets embedded, they will always be foreign to SE and rely on a ...
Links to Wikipedia help to give extra context to posts, providing information about the topics mentioned that is far in excess of what can be provided in posts. This can help the understanding of the casual reader.
status-completed, according to Marc Gravell on Meta.SE:
Thanks to @IlmariKaronen's excellent detective work, it looks like
refer[r]er plays a strong role here. The good news is that we've
already done the work to make markdown-mini emit rel="nofollow noreferrer" links (rather than just rel="nofollow"); this should
have the happy accident of fixing ...
We already have tag wikis for providing extra information about topics - this question was tagged with apple-ii, file-system and apple-pascal at the time that the edit was suggested. Relying on links to external sites for information is frowned upon because this site should aim to be self-sufficient - that's one reason why we don't have link-only answers....
Adding (Reddit) links is under all circumstances ok. It's a source and adding sources is helpfull to understand questions as well as answers.
And no, there is no need for a disclaimer. Any source on theweb can be changed and turned into something less desirable - 404 being worse than any NSFW content. After all, even a linked source, where the usefull ...
Archived Reddit threads on SFW subreddits are definitely OK. Threads on NSFW subreddits are definitely not OK. Unarchived threads... I'd say "yes with disclaimer".
However, if at all possible you shouldn't require anyone to click on any links. Quote relevant sections of the thread, including a link to the source. Here's an example.
When I provide a link to a resource I normally go to the Wayback Machine on the Internet Archive, save the page there, then link to the saved copy. Thus you're linking to the content that was intended and not any NSFW content that might show up later. It also prevents link rot.
No, and when the information is not widely known or reasonably obvious to current-day users we should document that in a Q&A pair and link to that instead. (See Curt J. Sampson's answer for examples.)
We're keeping all the necessary information on-site, not linking elsewhere for important information.
It's fulfilling our mission to document and ...