I think there's a difference between route designations that are of a sort that are not meant to used for navigation and only exist for internal administration*, and ones that are normal route numbers (in the range that signed numbers are, etc) that just happen to not be signed (perhaps appearing on mile markers).
Totally agree. Keeping up with all the unmarked state routes sounds like a nightmare. I vote to just map what's marked, especially if the unsigned route is a duplication of existing routes.
GA900 seems to be a signed version of the former. But so is, arguably, GA400 - the 4xx series were for hidden designations to refer to interstates, but GA400 never got given its interstate number to be the public-facing number, so the DOT's admin number got shown. That's a difficulty.
Georgia freeway numbers are listed on some state maps for reference purpose and on the Georgia DOT page, but except for where they are not an interstate, they are not marked in state and not meant as travel signs. No reason to include in my mind. Sadly, some map publishers are showing on maps, like GA 401 on I-75 and it drives me up the wall.
Similar to the 9xx series, in Georgia, the 7xx and 8xx series are meant for internal projects and are almost never signed. And yet again, because of one map showing one time 40 years ago, GA 754 remains on many a map in Cobb County despite that road never being signed as such. Another example of insanity to try to map all of those roads as the list is constantly changing as projects are added and completed.
I know North Carolina also is a state that controls every county road, but except on some street signs, I've never seen marked with a shield or highway exit signs. Would I like those mapped to help show every little place I've ever been? Maybe. But the level of effort to do that seems excessive for this site. I asked once about mapping county roads and was told that was too down in the weeds and difficult to do. Many of these unsigned state highways feel like the same and a bottom tier road.
I get the impression that making new systems is more fun than maintaining the old so there is a constant pressure to add systems. I get that. In my life and job, I hate the maintenance tasks like mowing the grass. Much more fun to plant new bushes. But at some point, all that's left to do is just mow the grass, trim the bushes and enjoy the beauty of what you have made.