CoyIM is not available in Debian buster or buster-backports.
https://packages.debian.org/search?keywords=coyim
CoyIM is in Debian unstable (sid) but somehow not even available from Debian testing.
Bug CoyIM in buster freeze up not closed.
coyim - Debian Package Tracker links to this bug https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=930332 as reason why it’s not in Debian testing.
Looks like the maintainer of that Debian package didn’t get to it yet.
user documentation added just now:
Instant Messenger Chat