I see that the coremod on CurseForge is cluttered with version numbers and yet is absent of critical versions like 1.5.x and 1.6.x

A suggestion is to simply put in as the latest of each sub-type. 1.19 is an oddball, and I believe that 1.16 is also unusual in this approach. Other than that, there is no reason or benefit to put in a supporting version as "1.20" when "1.20.1" automatically supports the version and vice versa.

Just fyi, as of version 1.4 on Curseforge, I only added the latest patch versions to the list of supported game versions.
Let's see how it turns out.

You might want to add in 1.19.3 and 1.19.2, because of the fact that they stand out in breaking compatibility versus 1.19.4.

is absent of critical versions like 1.5.x and 1.6.x

I intentionally didn't list these versions as supported on the 1.3 release as there are some compatibility issues with those currently.
We're currently preparing a new release with a fix for this (and many more) issue(s). So the next release will have older versions in the supported game version list as well.

there is no reason or benefit to put in a supporting version as "1.20" when "1.20.1" automatically supports the version and vice versa.

While I think that experienced players know this, I'm not sure if that's the case for the average Minecraft player.
But I'm definitely open to change that. Maybe we can collect some feedback on that topic here.

My question is: how many have a non-latest subversion of a legacy Minecraft build, and in particular modded?

On one hand, it seems sensible that a number of players may have for instance 1.16.4 or 1.12.1, due to forgetting or not bothering to update from when they created the instance. At the same time though, I am not sure how many have done so and have not moved on to later versions and sub-versions. This would, in my view, fall into a tiny portion of casual Minecraft players.