"In an information-rich world, the wealth of information means a dearth of something else: a scarcity of whatever it is that information consumes. What information consumes is rather obvious: it consumes the attention of its recipients. Hence a wealth of information creates a poverty of attention and a need to allocate that attention efficiently among the overabundance of information sources that might consume it." - Herbert Simon.
If we stipulate that the point of a guide is to point the greatest number of individuals towards the choices that will improve their chances of winning (as distinct from a character report, which can simply be "here's what I did"), then higher difficulty modes are the equivalent of more rigorous lab conditions for testing what's good or not. If we were designing a robot that needed to safely navigate an environment and we only tested in relatively simple environments, we may not be able to tell more resilient programs from more fragile ones. If we add more dangerous elements to the robot's testing environment, these distinctions will be easier to see. This is the value-add of testing a build on higher difficulty modes, and adding more guides that don't meet this standard consumes the same amount of attention (or more, if in a 2 hour video format) while distributing less rigorously tested information. It isn't saying that less rigorously tested information is necessarily wrong, it just hasn't proved its rightness to the same degree of scrutiny, and when I'm practicing good attention hygiene I prefer the higher standard.
Sheila already addressed your "metagame vs. presence of actual liches in vault" argument - the guide was written before lich was buffed, what was good or bad advice under a previous patch is not necessarily so under the current patch.
I do agree that guides can include explaining where fun but suboptimal pieces can fit into a build. Someone (bpat?) came up with the "core/luxury" distinction of points spread, and I like the trend in guides that is pushing them towards "here is the minimum stuff you need to get in order to get the quality stuff from this class" (see Cathbald's Shadowblade guide
) rather than telling you where you have to spend every single point (although I think Adventurer builds are often an exception to this). I just think these things should be labelled clearly, and a more rigorous lab again lets the guide author have a better sense of what was critical and what only optional, even if fun.
I also agree the game needs, and has, classes that have different viability on different difficulty modes at different player complexity rates. But this information seems like the kind of information that is critical to know, and place in a guide, rather than handwave away - if e.g. you can't win on Insane with Bulwark, better to find that out, post it, see if anyone else has better advice, and convey that to readers of your guide, than not test it and disseminate advice that didn't lead you to a win.