Hyperspace It's increasingly common. I had to replace newsflash with newsboat a while ago because it used some weird webkitgtk package version that for some reason lists systemd or libsystemd as a dependency.
High level userspace packages should always depend on systemd optionally unless there is no other way to do it. They don't actually need the init system, as very little of userspace ever does. But there's either a deliberate effort to put systemd in everything, or people just forgot how to manage dependencies sanely as from a software architecture standpoint.
nfg I have xdg-desktop-portal but only as a transient dependency for something else, and I don't think there's anything on my system that uses the desktop portal APIs. But if I see something I'll report it. Nice work though, this one was hairy. I tried to sanitize this one myself but ran out of time and patience midway.