Ubuntu versions don't differ much.
I am by far not an Ubuntu expert and I know that there are significant differences between a Ubuntu version released in 2009 and one released in 2019. Having used older versions in a VM I can think of several problems that stem from using an older version, when attempting to upgrade the installation, to a newer version.
There are also upgrade limitations when dealing with LTS releases, for instance, Ubuntu recently in the last 2 years, stop providing a 32-bit LTS version.
Solution that's working in Ubuntu 11.04 has chances that it would work on 16.04 too unless it's a bug.
It would be up to the person answering the question to determine if the problem described is due to a bug or if the solution applies to the version of Ubuntu the author is using.
One of the first things I do as somebody who reviews a lot of contributions, to verify if an answer applies to a specific version of a program, I typically stick to operating systems I know though.
If they don't, we can always ask the questioner to mention their Ubuntu version.
Even if the tags are kept this should be done. Tags should not be a way to provide information that isn't specifically stated.
Multiple tags can become an obstacle in earning that golden tag badge
This isn't a valid reason to get rid of a tag. If a question applies to multiple versions, as an editor you can suggest an edit that adds the ubuntu tag to the question, in addition to fixing any other problems with the question.