I think responses here will somewhat depend on the topics we frequent. Some topics are much more version-dependent than others, which influences our thinking on the relevance of version information.
Even with versions that are similar, there can be differences in the UI, or configuration details, or replicating the problem. With applications like Excel, capabilities have been added in successive versions, which may drive the choice of solutions, so it's important to know which version the OP is using. In general, adding the specific version can't hurt unless it's a generic question known to be not version-specific.
We do often get questions that specify version information, and years later, a reader will ask whether the information is still relevant to their version. We also get questions that don't specify a version, and years later, a reader will ask whether the information is still relevant to their version. Specifying the version doesn't really make questions any less relevant. In fact, it's useful to have old questions revisited to see whether there are new solutions.
In cases where you come across an old question (including your own), that you know is still relevant to the current version, it can't hurt to add a comment to indicate that, or even edit the question to add a supplementary note saying that it is still applicable at version X. Similarly, if you know that it is no longer applicable to current versions, and that isn't already discussed, you can add a comment or edit.
People sometimes post an answer stating that the problem can be solved by installing the current version. If the question was version-agnostic, that's fine. But if the question was specific to an earlier version, that's more in the nature of comment fodder since it's not a solution for the earlier version.
The question is really more "where" to put version information, and sometimes "when", rather than "if". The tags and title serve to attract the people who think the question is relevant to them and/or can answer. Note that the tags should be, and generally are, at a granularity where they reflect substantive differences. For example, we don't have a tag for every version of Firefox, but we added the firefox-quantum tag when the code base changed, and things before and after are very different. The body of the question needs to contain the details necessary to propose a solution.
Some reasonable guidance:
If you know the the problem is version-specific, use the version-specific tag (only; as in don't also include the version-agnostic tag), and think about whether it would be helpful to include the version in the title (but don't use a "title tag", like Windows 7: yakkity blah problem). In some cases, like Firefox Quantum, the firefox-quantum tag covers a range of versions, so it doesn't hurt to also identify the specific version within the question.
If you know the problem is specific to several versions but not others, use tags for the relevant versions (only). It can't hurt to also mention the specific version you're using within the question.
If the question is about a very old version that is no longer in mainstream use, especially if no longer actively supported (e.g., Windows XP), use the version-specific tag, and it doesn't hurt to mention it in the question, perhaps even include it in the title.
If you think the question might be version-related but aren't sure, use both the version-specific and version-agnostic tags. Include the version you're using within the question.
If you have no idea whether the problem is version specific, use the version-agnostic tag (only), and identify the version you're using within the question. Answerers will identify whether it is version-specific.
If you know that the question applies to all "recent" versions but not "archaic" versions (e.g., Windows since version 7), the archaic versions are like exceptions; people realize that they were different, they are no longer mainstream, and people won't assume that the problem or solution applies to them. Use the version-agnostic tag (only). If you're sure the specific version isn't relevant or the question is generic, no need to mention the specific version in the question.
For an application like Excel, this would apply to a question about a specific fundamental function that has been present in all versions, or at least all versions likely to still be in common use.
