Why do you find it so odd?
The whole addition of the string “-nightly” should at least have made you suspicious about what that means, and it is a common term for software that is under heavy development.
One of the common properties for software that is under heavy development is that some features do not work properly, or are bluntly absent at all.
I think the file format is now frozen (KiCad is getting close to a V6.0 release) Having a file converter for a file format that is still changing is a lot of extra work, and there are plenty of things that developers of KiCad find both more important and more useful than this. But still it’s possible someone already started on a module (in probably Python) to read the new file format for use in scripting. (And a module for writing in the old format probably also exists.) If you’re good at Python those are probably relatively easy to knit together.
On top of that. Because KiCad is an open source project, everybody can update to the latest stable version for free. This means that there is generally much less need to write older versions of file formats.
Two related links from the FAQ section:
How big is your schematic?
Designing a schematic is much more work then re-creating it, especially when it was your first schematic in KiCad and you also had to learn to use Eeschema itself for the first schematic.
You also wrote that you had just made your first iteration of the schematic. Maybe it is a good idea anyway to redraw it, and do some cleanup at the same time.
Just for completeness:
For the PCB, there is a partial way to convert to any KiCad version, even if the orignal PCB was not made in KiCad, by opening the Gerber files in Gerbview, and then: Gerbview / File / Export to Pcbnew