I have a weakness for nostalgia and some interest in Unix history, therefore I would like to know which are the oldest still usable irc clients that can be run on Linux. In this context I value age over performance.
It depends on your definition of "usable", but... a version derived directly from the very first irc client and server (the page only mentions the server, but the client is present in the download) can be found at https://people.symlink.me/~rom1/projects/oldirc/?view=list.
It builds and runs on my system just fine (this is a release specifically modified to run on modern Linux systems), but with some caveats:
The protocol it uses is notably different from modern IRC. Channels are numbered rather than named, the client can only be in one channel at a time, and the protocol-level command to join is
CHANNEL rather than
/join works on the client.) Attempting to connect to a typical modern IRC server (which otherwise might theoretically at least allow private messaging to work) runs into problems since it doesn't answer a CTCP
It can connect to its own server (you must set up
/usr/local/etc/ircd.conf for the server to work and accept connections), but getting it to connect to a modern server would probably take a significant amount of work, with even more modification required for being able to support being in multiple channels etc.
For a usable client set up for the modern protocol, it's likely the oldest one is ircII.