libc-shm-shim is snap intended to be used when building snap which is likely to use multi threading trough Python, which eventually ends up denied because of wrong syntax of file created under /dev/shm used by libc semaphore.
Solution is to tweak libpthread library, which this snaps delivers.
There can be indeed part in snap which does recompile libc, but this is often unnecessary and time consuming, so having snap with ready to use library is convenient way.
Library takes env variable SNAP_SHM_PREFIX or SNAP_SHM_SUB_DIR to use sub-dir or add prefix to file names. So it can be reused in any snap and controlled by env variable.
Reason for track request is to have this available for UC16 as well as UC18 environment.