The status of abandonware is dodgy right now, and I assume that SE doesn't want to be a repository for aging and quasi-legal links.
Not sure how to resolve that, but I want to stress that even if we disregard legalities, such answers would often be low-quality.
That being said, there are a few well-known informational sites out there that have been around for a long time. References to those as a place to start would be, IMO, appropriate.
Ok, Let's Talk About This
In general, SE sites assert that link-only answers are bad for SE. The reasoning is that they are not good answers, and we want good answers. Now, a good answer has many qualities (and we can discuss these in detail), but a main quality is that it should continue to benefit others in the future, and it should not be particularly time-sensitive (to some extent.)
Questions that are resolvable primarily with a set of offsite links everywhere else on SE are often "answered" in the comments, with a suggestion of web search terms. These sorts of questions are consider poor quality by most of SE, and I tend to agree.
SE sites aim to be relatively stable repositories of good questions and answers, and answers where the information is primarily or only offsite links are not stable. Whether an answer has utility and usefulness now is only one small part of what makes SE sites good. And we need to recognize that, no matter how much we want to preserve and share these resources.
So, this is the rationale, and each site has to adjust this knob to suit. So, how do we try to align with the spirit of the SE charter and construct good answers that satisfy this problem?
The problem at hand, as posed in the question, is how to share some of those links we've collected in our bookmarks for books, magazines, manuals, firmware images, ROM images, and even abandoned commercial software? (i.e., we are not talking about answers that use offsite links to augment the information provided, or provide examples. This is, of course, totes legit.) It seems reasonable. What's the problem?
But it is a problem, for at least two reasons:
As discussed above, link rot has made some of those bookmarked links bad already. Domains expire. People die and no one takes over (this nearly happened on a TRS-80 site I know about). Sites get re-organized so deep-links go nowhere. This is not a problem strictly limited to SE, of course, but it has to be recognized as a problem. It is the source of many bad answers, where "bad" is defined as "potentially useful now, but unknown to be useful at any time from now."
Whether we like it or not, much of the material we want to share is, technically, not in the public domain or otherwise freely available in a legal sense. It is copyrighted or trademarked or otherwise is intellectual property owned by someone else (even if that ownership is unclear). Since I am not a lawyer I am not going pretend I know the details of the legalities. But even if this stuff is shared easily, and no one seems to care; and even though linking to a site that shares copyrighted material is not, in itself, illegal; the fact remains that we are in interesting legal territory, and SE sites want to comport themselves in a reasonable (and uninteresting, legally) manner. I'm not suggesting no law ever be crossed, ever, but that there is a legal aspect to all of this (which in itself is a subject for a good Retrocomputing Q&A, now that I think about it.)
What Can We Do?
Neither of these things are intractable, but they offer challenges.
So, what can we do? Here are some things I have been thinking about that are presented as a way to open up the conversation:
I raised this in another meta discussion. It is a way to have a rotating banner of interesting sites, which could include sites that hold caches of interesting technical documents. It could include those sites that aid you in finding ROMs or firmware or software. (I'm not talking about linking to the offshore ROM download sites, but rather the hobbyist sites that we should want to preserve that show techniques for finding such stuff, and using it.) Think of it as a curated list of known good sites that may offer the typical Retronaut another avenue of exploration.
I don't use SE chat very much, but I know that other sites use chat as a way to discuss ephemeral and ad hoc subjects. I believe chat is archived, so it can be treated a bit like a forum (i.e., aging poorly, but offering historical hints for spelunkers). For example, I have a huge cache of TRS-80 Model I and Model 100 catalogs, manuals, and magazines stashed on a DropBox type service that I certainly wouldn't share in an answer, but would probably offer in a chat.
I am continuing to think about this, but at the end of the day we have to comport ourselves in the same manner as any other SE sites, especially during beta. But we want to preserve some of this information if we can, but without filling up the site with dodgy, junky answers.
The primary concern of SE is to provide good answers, so I still maintain that should be our primary concern, too.
But I'm open to suggestion!