With libcap-2.28 we started to include a mini library: libpsx. Through some linker magic, libpsx provides a way for pthreads to all share kernel security state - it provides an extension of the nptl:setxid support internal to glibc.
Currently, this library is only built as an un-versioned static ".a" library. Empirically, the API is still evolving, and the ABI provided by this library has changed (backward-incompatibly) in the libcap-2.29 and 2.30 releases.
For now, I'm reluctant to offer libpsx as a versioned ".so" file because I want some API stability confidence in the face of real world usage. I may also need to adjust the API further if the Go package requirements on the "libcap/cap" package from the upstream golang sources, change dramatically. That is, the eventual state of https://go-review.googlesource.com/c/go/+/210639/ .
That being said, at some point, the API will stabilize and producing "libpsx.so" will make sense. I'll tentatively set a date of 2020-10-10 as a rough timescale for adding this support. But will fine tune this date the closer we get to it.
Adjusting this to being classed as a possible enhancement.
I've started to look at this.
This should be fixed with the following patch (which will eventually be included in 2.45):
I'm going to mark this feature request fixed, but feel free to confirm, or raise an issue via this present bug.