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).
The latter sorts of routes are the sort of thing that are worthy of the database. The former forming a lot of the argument against adding any. Distinguishing between the two is something that needs to happen.
Stuff like Florida's state designations for US routes and Interstates are the latter - they form an integral part of the grid of state route numbers, but stuff like UT900 that have been deliberately numbered with out of sequence number are the former.
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.
*sometimes these might be signed as if they were meant to be signed, even though they weren't. eg
here. C road numbers are explicitly not meant to appear on signs, but merely be for administrative purposes, but one is signed here on normal direction signage (they often appear on temporary signage, and sometimes on regulatory signage).