XChat was once the most popular of the X11 clients, but by 2016 has fallen into abandonment - the last meaningful releases were in May 2010, the user forums have been taken offline, and some distributions have begun to remove it from their repositories as unmaintained software. In it’s present state, we can no longer recommend XChat.
Hexchat is a fork of XChat that is actively maintained, with a substantial following. For all intents and purposes, it’s the spiritual successor to XChat, and for most users, likely to be an easy replacement.
So… what Happened?
At the height of its popularity, XChat was widely included in Linux distributions, and had an official Windows port that was gaining ground rapidly on mIRC. It was the Windows port and decisions around it by XChat’s maintainer that would prove fatal. XChat was released under the GPL license, a copyleft free software license. The maintainer unilaterally declared that the official releases of XChat would be shareware under Windows, citing the rising popularity, complexity of development, and cost of maintaining development tools for Windows. The backlash from this led to a number of unofficial ports, the loss of virtually all outside developers, and most importantly, the loss of the community’s trust - even among Linux/Unix users who were not directly affected by the change.
One of those unofficial Windows ports, HexChat, grew up into a not just a port, but a viable, active, and well-maintained fork of XChat, and the community of users and developers have largely followed.