On Ubuntu since 18.04 (Bionic) and Debian since at least 9 (Stretch), the dh-systemd package has been a dummy package without any actual purpose (the package only contains a copyright file and a changelog.gz file).
I believe the only distro we currently build for that has an actual dh-systemd package is Ubuntu 16.04 (Xenial), which is about to be deprecated after the upcoming 10.5.10 release.
More importantly, on Ubuntu 21.04 (Hirsute) the dh-systemd package has been completely removed, so MariaDB fails to compile there because of a missing dependency. Hirsute is only 10.5+, so the change only needs to go there.
So it is time to remove this package from our build dependencies. This could be done easily after the 10.5.10 release because after that we will be removing Xenial and we will then have no deb-based distro that actually has a real need for that package. If we need to provide Hirsute packages with the 10.5.10 release, we could still remove it, we would just need to have a workaround so that at least Xenial does still require it.