Yeah, pretty much all the flat ride reskins - or at least the decent ones - are really old. I don't know of any new custom flat that has been made in the last 10 years.
I think part of the reason is that in vanilla RCT2, flat rides were limited to 4x4 at most, which is really small and just doesn't look good for most flat rides. You were also limited to reskinning an existing ride - your ride had to animate the same as something that already existed. So custom flat rides just weren't that appealing. Shoestring rides don't look great but they can at least be a reasonable size.
With the new save format in OpenRCT2, it's now possible to work around these limitations but there just aren't many people working on custom rides. All the new coaster types added to OpenRCT2 so far were made by the same person.
However, there has been some work done on it. One person has made significant progress on a custom Huss Condor ride so perhaps that will eventually make its way into the game.