mcrouter is currently only available for Bullseye:
fnegri@apt1001:/srv/wikimedia$ sudo reprepro ls mcrouter mcrouter | 0.41.0-1 | buster-wikimedia | amd64 mcrouter | 2022.01.31.00-1 | bullseye-wikimedia | amd64, source
We need to install it in Bookworm as part of T341285: Upgrade cloud-vps openstack to version 'Antelope', and I imagine other teams will need it in the future.