Will there be any development builds for older OSes?

[Bingo]

I'm running 10.13.6 (the most recent my Mac can run), and I'm a bit envious of the folks that get to try the new builds I'm hoping to soon be able to take advantage of the "force export" feature when a pool hangs, so I can avoid reboots.

Are the builds very cumbersome to make? I'm glad builds are being made at all for older OSes — it's rarely a priority for macOS developers across the board. But it'd be awesome every once in a while to have one of these developer builds

Or, can I build one myself? I only see download links for builds...

[tangent]

I'm not an O3X developer, but my understanding is that the OS version in the package names is for the oldest macOS kernel it will work with, not the specific kernel it must run with. There may be one that runs on your system even though it isn't specifically targeted at it.

If not, then you can build from source. The old build instructions are here, but beware, they're pretty misleading now, having last been updated over three years ago, before the big merge of the OpenZFS forks. I provided some updated build instructions here, but those haven't been folded into the wiki yet.