.htaccess is well discussed on StackOverflow's meta, in particular there are a lot of discussions about whether, and exactly when, questions about
.htaccess and URL rewrites, and so on, are on-topic. On the other hand, I've been seeing
.htaccess questions more and more on SuperUser as of late – enough for me to notice it – and I see no discussion about it on this meta.
I usually will answer question on "how do I configure outlook?", "How do I move this ost file?" "How do I print a hundred files to PDF without opening and printing each of them?". I do not see how "How do I configure a webserver?" is all that different.
Mind you, complex rewrites might be better off on Webmasters. As would some of the unix question which might be better off on Linux&unix and some of the apple questions on AskDifferent. But we do not kick those to another site just because they are also on topic there.
An I guess I just answered in the last sentence. Also on topic. So yes, I think they can be on-topic and this the proposition they are never true on [su] does not hold.
They certainly shouldn't be moved en masse to serverfault. Unless they're related to professional admin work (and most professionals won't use .htaccess for quite a few reasons I don't need to dilate on here), they don't belong. Not everything related to a server is allowed at SF.
If they're on topic on SU, they should be answered there; webmasters.se might like them. I wouldn't migrate them to SO as generally they have little to do with programming.
A ton of them tend to be duplicates, so you might want to canonize some questions to cut down on the repetition.
I would say they boil down to configuration questions like any other. Yes, Webmasters.SE is more focused towards that area, but we likewise get (and accept) many questions on general *nix configuration that would be more suited towards Unix.SE. General policy is to not move questions where they are on topic on both sites, unless specifically requested by the asker.
Generally, if it's related to a computer and not about programming (or software-rec and related), it's on-topic. We also specifically exclude video games and webapps/services, but not configuring your own server.
If it is on topic in the Help Center, it's on topic here! There are topics that are allowed on more than one site. The community (or at least, I do) tends to migrate questions that seem to be about just server environments, where they may be using it for multiple servers or specific configurations.
If it's for a home server, then it's usually on topic here. I at least wouldn't migrate certain questions over. If there is a question in particular, feel free to add it to your question.