Bionic C Library

Which C Standard Library is used by Xen/QubesOS? Would it be possible to request

that the Android Bionic C library is used?

glibc

I am not in a position to tell. But the whole ecosystem is very much dependent and relies on glibc. And it is a proven technology. Any change to Bionic or Musl or similar would be most unlikely.

Bionic and Musl are unproven C libraries; Bionic is Android no?

dom0 uses Fedora, which uses glibc.

Proven on the current overall status of the project on x86 platform. And Qubes uses Fedora and Debian. And they are glibc based. If Qubes OS was Android based, it could have used Bionic. If it was Alpine based, it would have used Musl.

Are there any Linux distros that use bionic?

I am not aware of any.

I only see builds for Gentoo and Ubuntu 18