Is this usage correct? “versions N and prior”

I'm thinking without the 's' is correct. This forum post talking about adjectives states,

The simple rule is that the form of adjective doesn't change based on gender, number, and tense.

And this blog post says,

As a teacher, I have always taught the rule that there are no plural adjectives in English

Although the second article goes on to say American English neglects to pluralize adjectives while British English does, and how American English is slowly picking it up.

So my vote is the word, 'version' is being used as an adjective to describe the different software packages and therefore should be treated as such, in this case, without the pluralization.

I think either usage would be correct, but I would lean towards "versions" for clarity.

Another example is this:

Suppose you had four boxes, marked A, B, C and D. Two of the boxes contain prizes, the other two are empty. You might say

boxes A and B both contain a prize

or

boxes C and D are empty

In each case you are referring to a group of boxes, so you would use the plural form of box.

You could read

Version X and prior

as

Version X and prior [versions]

The word versions is implied in this case. This may be idiomatic rather than strictly grammatical, but I think the reader would understand.

Regarding the colon, it wouldn't be correct to use it within a sentence, but would be fine in a list.

If I read:

Version N and prior

I would actually read it as:

Version N and prior [versions]

rather than (as you suggest) as:

Version N and [versions] prior

With that in mind, I would say that Version N should be singular.
Personally, I would probably use "earlier" instead of "prior":

Version N and earlier [versions]

But the easiest solution to overcome the issue of whether to use singular or plural, is to abbreviate Version(s) to Ver.:

Ver. N & earlier

You do not say in what context you are planning to use this expression, but I'm making the (perhaps rash) assumption that it is in some kind of programming context. If that's correct - and although I wouldn't want to belittle the need to use the singular and plural forms correctly - I would suggest that, in that context, your readers are not likely to give the matter a second thought, if indeed they even notice the issue.