The Tor team unveiled its Messenger app in 2015 to boost the security of existing chat clients, but those plans are coming to an end less than three years later. The developers are ending support for Tor Messenger due primarily to a lack of support. The developers behind Instantbird, the foundation of Messenger, have stopped working on the interface. There wasn't much overall progress on Messenger, for that matter. The app was still stuck in beta testing years after its debut, and the creators had to ignore bug reports and feature requests due to the limited resources.
There were also underlying security issues. Most of the messaging apps Tor Messenger supported are based on client-server architectures, and those can leak metadata (such as who's involved in a conversation and when) that might reveal who your friends are. There was no real way for the Tor crew to mitigate these issues.
There aren't many viable alternatives. Tor suggests CoyIM, but it's prone to the same metadata issues as Messenger. You may have to accept that a small amount of chat data could find its way into the wrong hands, even if the actual conversations are locked down tight.