After upgrading one of my physical hosts to Debian Trixie, it failed to boot.
It complained about the service for mounting my encrypted disk not starting.

Turns out this is a known problem (even mentioned in the Trixie release notes).
For cryptsetup to work in Debian 13, the systemd-cryptsetup package must be installed:

# apt-get install systemd-cryptsetup

After doing this (via the rescue shell) and rebooting the host, the system started seamlessly.
And the encrypted disks were mounted as expected.