I feel it's appropriate to keep the tag for that distribution. But what does the community think? Should we have tags for Ubuntu, Arch, CentOS, etc?
It is extremely rare for Linux software to require a particular distribution. I've seen a lot of questions with the ubuntu tag and retagged them to linux, because the questions were not about Ubuntu, they were about Linux.
If the question is “I want software that does X and runs on Linux, and by the way I'm using Ubuntu”, then Ubuntu is not a key requirement, because any Linux software will run on Ubuntu. “Available as an Ubuntu package” would be a plus, but that doesn't warrant a tag.
The ubuntu tag should be used in addition to linux only if the question does not apply to every Linux distribution. For example it would make sense to use ubuntu if integration in Unity was a requirement.
Most Linux distributions will not warrant a tag at all. Ubuntu does (and even then it's overused) because it provides an integrated environment, and it makes sense to have requests that are somewhat specific to Ubuntu in that they require good integration in that environment. centos or debian or arch-linux wouldn't make sense in this way.
See also How specific should the OS tag be? for a more general perspective.
Most definitely not!
Tag the OSes generically – windows osx bsd android windows-mobile windows-phone iphone etc. or web or cross-platform – and then mention the specific flavours (like a Windows® version, or a GNU distribution) in the post body.
Answers are generally relative to the operating system, but mostly alike across different versions/flavours of one OS (such as Desktop Windows, or GNU/Linux). Some do have minimum requirements (such as Windows >= XP, or Linux Kernel 3.2+), but that’s okay.
While the distribution can influence the software selection, it does so on a much smaller scale than the operating system. Tags are for quick filtering, the type of distribution is only a minor clarification, and usually (yes, not always, but usually) not even needed.
I've posted a question and tagged it with both Linux and Ubuntu - Linux for the broad OS category, and Ubuntu for discovery via search engines (notice that the page title starts with 'ubuntu') when you tag it as such). Since many people include the distribution in a software search*, this could be useful for SEO purposes.
But if we care most about the broad OS categorization and want to mention distributions just in the title/body, I think we should consider changing the tag to something like 'ubuntu-specific', or even removing the tag alltogether. Or we should accept that a lot of re-tagging is going to be needed, because I expect many Linux/Ubuntu questions will get just the Ubuntu tag from the poster.
* Statement based on my own search behaviour as well as viewing many such questions on various fora, superuser, etc. ;)
The linux tag itself is too narrow, I suggest we retag most questions in this tag "unix". People don't often ask for software run with linux specific syscalls. People often ask for generic graphic and commandlines. The linux tag stands when it comes to firewalls, filesystems
However the ubuntu tag can keep as is, since we have for long tolerated a seperate askubuntu stackexchange. This way people who ask for the ubuntu sector are prepared to get ignored by people not wanting to give an "apt-get" answer.
Yes - This isn't just because of the difference between desktop distributions like Ubuntu and Fedora. It's also because of enterprise distributions like CentOS and Red Hat. The software that's easily available for one distribution, especially in the enterprise sector, can be very different from other distributions.
This also applies to people who might, for some reason, be using CentOS as a desktop distribution (This happens at offices I've been to, for example) and they want to replace Gnome 2. Unfortunately, running Gnome 3 on CentOS is nearly impossible.
Additionally, software that is recommended may require dependencies that are unavailable for a distribution. CentOS is another example here - there's no real easy way to install
mono, so I had to run my Kerbal Space Program multiplayer server on an ArchServer.
This demonstrates, at least, that the distribution that a user has can influence the selection of software he can install. Even though most Linux software will compile and run on essentially any distribution, it's still enough reason to keep the distribution-specific tags.