Recently, I've seen almost every question where the OP is having a problem caused by using an EoL version of Ubuntu being closed as a duplicate of this:
How to install software or upgrade from an old unsupported release?
Useful as this post will hopefully be to people who are using an unsupported release, I don't think we should be closing these questions as duplicates of this one unless they are actually asking something like "How can I install software or upgrade from an old unsupported release?"
As an example, this question*: https://askubuntu.com/questions/1006572/unable-to-locate-ubuntu-restricted-extrasthis question is totally not asking how to upgrade from 17.04. It's asking how to install ubuntu-restricted-extras*. We should certainly close that question; in fact we have a whole close reason for new posts that are specific to EoL releases
This question is specific to an end of life Ubuntu release. These are no longer supported and are therefore off-topic here. To upgrade, see: How to install software or upgrade from old unsupported release?
Notice in particular that this close reason even contains a link to that useful post...
I think it would be a lot less confusing to askers and other visitors if we actually used the correct close reason for such posts and avoided closing stuff as duplicates of things they are blatantly not duplicates of.
**I now realise this is a bad example and closing that question as a duplicate was reasonable thanks in part to You may be wondering why I'm complaining about this question when I myself am in the list of close voters there; due to an incredibly annoying feature of how close reasons are displayed, it looks like I voted for this dupe target because that's what the majority of voters did, when actually I voted to close the question as EoL.guntbert's answer to this question. I have been prompted to edit this post by some discussion in chat.
While I think it is fine to close questions as off topic if they ask about installing software on an old unsupported release, I do realise that it's often at least equally reasonable to close them as duplicates of How to install software or upgrade from an old unsupported release?.
What prompted me to write this question was seeing a wider variety of questions specific to EoL releases being voted to close as duplicates. In general, these were package management issues (this one might be a reasonable example), so I think it's possible to argue that they could be closed as duplicates of the canonical question. This means I have changed my position slightly from when I originally wrote this post.
Still, I prefer such questions (about general package management difficulties) to be closed as off topic, because I think it's confusing to close posts as duplicates when they aren't very similar to the target. Certainly, questions about EoL releases that are not about installing software or upgrading to a new release should be closed as off-topic rather than as duplicates.
I should also mention as I did in this other meta post that closing questions as duplicates causes them to not be deleted nearly as easily as questions closed for other reasons. If the questions are useful, that is a good thing, but if they are misleading as search results or have bad answers, it's a bad thing. This is another factor that leads me to prefer closing low quality questions as off topic rather than as duplicates if there is a choice of reasons.