Matrix Synapse is indeed not available in Debian 11. I believe, this was a conscious decision by the Debian packager due the changes happening in Matrix ecosystem + Debian stable’s nature.
However, matrix-synapse is now present in bullseye-backports, so matrix-synapse should be installable if one is on bullseye and has selected Frequent Feature Updates (i.e., backports) (System/Update page). Presumably Frequent Feature Updates will be needed to install matrix-synapse for the life of bullseye as stable.
I forgot about apt preferences and I see matrix-synapse pinned to buster-backports in 51freedombox-apps.pref, but I somehow ended up with v1.42 from bullseye-backports when I went through auto-update from buster to bullseye:
matrix-synapse/bullseye-backports,now 1.42.0-1~bpo11+1 all [installed]
Yeah, this works because there is no matrix-synapse in bullseye itself, so it will pick the package from bullseye-backports, even without the preferences file updated.