I guess I don't find this clearer. Users are going to wonder what the N and D stand for.

Would N/A be better?

Or "Unlimited" ?

Unlimited is clearer, but kind of long.
What about something like "Any"?

Didn't we used to have "Any" and we stopped? Ah, yes #1866 and related #1882 and #1887

I might not get this right, but as I see #1866, the problem was with the mod-JSON parsing and handling "any" and NULL differently?
My PR would just change the displayed string in the GUI, so this shouldn't affect the core.
I got no errors during testing (-> sorting by max ver. column and install some mods), but please corrcet me, as this is a bit more complex for me.

Yes, I think in between we changed the display and sorting values for this column to different values.

I do think "any" would be preferable to "N/D" (which I admit I am not instantly parsing, myself). Not defined?

Yes, N/D stands for "Not Defined", but if we already have trouble understanding it, it's probably not a good idea. So, "any" now?

If you can push a new commit with that change in the next 12 hours, I'll include it in the 1.25.1 release I'm going to put out in the morning.

Done!

Might I suggest that you make sure this works EXACTLY as the current KSP-AVC works (which I'm now maintaining)? Some bugs were fixed which might affect this

Not saying this is wrong, just want to review it together

This shouldn't affect AVC at all, since it just changes just the representation of an empty maxVersion string in the GUI.

Might I suggest that you make sure this works EXACTLY as the current KSP-AVC works (which I'm now maintaining)? Some bugs were fixed which might affect this

That's a completely different bit of code (assuming you're referring to the parsing of max KSP from .version files)

empty to "any"?
Then maybe 9.99.99, 99.99.99, 1.99.99 also to any?

KSP-CKAN/NetKAN#5356