Post #14 explains a new mechanism by which systemd can now dynamically add entries to the systemd-boot startup menu, for troubleshooting and so on. This feature seems mainly for VMs.

Upstream changes force users to pick an init system at install instead of boot Debian 13 "Trixie" is coming, closely followed by a new release of MX Linux, MX 25, which will lose some of its ...

It’s straightforward to start the systemd unit that fires up console on /dev/ttyUSB0 and connect to it, log in, etc…. But I don’t get any kernel output there even after boot up.