Package kernel-headers-un-def: Information

This package makes Linux kernel headers corresponding to the Linux
kernel package kernel-image-un-def-5.10.209-alt2 available for building
userspace programs (if this version of headers is selected by
adjust_kernel_headers).

Since Linux 2.6.18 the kernel build system supports creation of
sanitized kernel headers for use in userspace (by deleting headers
which are not usable in userspace and removing #ifdef __KERNEL__
blocks from installed headers).  This package contains sanitized
headers instead of raw kernel headers which were present in some
previous versions of similar packages.

If possible, try to use glibc-kernheaders instead of this package.