mIRC should be supported by any network which runs on the IRC (Internet Relay Chat) protocol. (Note: mIRC is just a client, not a chat system in and of itself)
IRC has been around for a very long time (hey, I first used it a decade ago... coming in by raw telnet in the days before mIRC even existed) and there are various flavours of it. It is continually evolving, there are literally hundreds if not thousands of public networks (searchirc.com regularly records a total of over one million users concurrently on the networks it monitors) and it is therefore very unlikely to "die out" just because it hasn't been assimilated by Windows.
PastMaster
