kakoune.org has a list of community resources. Under the Matrix option, it sent me to https://matrix.to/#/#kakoune:libera.chat which does not appear to exist.
A lot of Kakoune users hang out on IRC, on the libera.chat network, and for a while libera.chat provided a bridge to Matrix, so Matrix and IRC users could transparently chat with each other. Unfortunately, a few years ago that bridge was permanently decommissioned, with no replacement, so that Matrix room no longer exists.
Somebody should probably remove it from the website, yeah.