People really need to stop saying this rather than taking the time to actually understand what people are objecting to.
Lowercase is a UNIX convention. SystemD does not have a UNIX approach.
Since SystemD is not a Unix service daemon it shouldn't be spelled "systemd" to prevent confusion.
Should I pejoratively call you "CRaIg" because I think you don't match up to my standards of what a Craig, or even a craig, is? Is that okay?
I don't think it's okay. Names have meaning. It is a really minor bit of respect to use them correctly.
* https://lists.debian.org/debian-devel/2016/08/msg00547.html
Moreover, the term "System D" is somewhat synonymous with the term "black market", which Wikipedia defines as a: "series of transactions that has some aspect of illegality or is characterized by some form of noncompliant behavior with an institutional set of rules."
That is a surprisingly good analogy for how systemd acts on a machine, but there is another analogy to a similarly worded term: "Substance D", from the novel "A Scanner Darkly". In the novel, it is described as an addictive psychoactive drug, also known as "Slow Death". Credit for that comparison goes to this foundational criticism of systemd:
systemd (not SystemD) changes a core part of the OS, we go from a userspace being bootstrapped by a bunch of different shell scripts calling a dozen individual binaries, to an actual system and service manager configured with unit files.
It's a large change, although one other operating systems went through a decade earlier, and some people don't like change.