The entirety of GNU Parallel is just one Perl program. It could be copied over and used in a pinch. The installation itself is very simple and no special dependencies or privileges are needed.
On all such systems, it is very easy for the user to install any missing POSIX utility, but it is also easy to install any non-POSIX GNU utility.
So not even xargs is certain to exist by default on all systems.
Moreover, POSIX xargs is restricted to execute sequentially all processes.
Any use of xargs for parallel execution is non-POSIX, so in that case there is no reason to not use "parallel" instead.
parallel --embed > parallel.sh
Then store that in your source repo and use it wherever shells are used! $ parallel --embed > parallel.sh
Unknown option: embed
[edit] Ran it in Ubuntu 22.04, it does output a bash script ... which still depends on Perl.