> 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 [tag:ubuntu] tag to the question, in addition to fixing any other problems with the question.