Sorry, the first message accidentally got sent while I was still writing it, intended full text below:
Installing systemd 240-1 breaks plasma-workspace: xinit /usr/bin/startkde hangs with a black screen, with kdeinit5 stuck at 100% CPU usage. Attaching strace to kdeinit5 reveals that it's trying to close bogus file descriptors: […] close(39936688) = -1 EBADF (Bad file descriptor) close(39936689) = -1 EBADF (Bad file descriptor) close(39936690) = -1 EBADF (Bad file descriptor) close(39936691) = -1 EBADF (Bad file descriptor) close(39936692) = -1 EBADF (Bad file descriptor) close(39936693) = -1 EBADF (Bad file descriptor) close(39936694) = -1 EBADF (Bad file descriptor) close(39936695) = -1 EBADF (Bad file descriptor) […] Reverting to systemd, libnss-systemd, libpamsystemd and libsystemd0 version 239-15 fixes the issue. MATE is not affected, so it actually may be a Plasma issue that systemd just revealed.