You did, probably just didn’t see it ;) It’s been part of it for years, since around 2020 according to GitHub. But to be fair, calling the option “Profile” might not be very intuitive for some people, so it’s easy to miss.

No need to manually install desktop environments, archinstall also does that (Profile --> Desktop).

Plasma’s shake cursor plugin is a pretty recent addition, according to KDE’s GitLab it originally got merged just 10 months ago. Enabled by default since 6.1 (June 2024), with high-resolution cursor coming shortly after that iirc. So it’s basically the same as on macOS now, but only since a few months. I don’t know exactly when macOS introduced it, I’ve read somewhere it was with El Capitan, so that would be 9 years ago. Either way, macOS definitely had it first.