CHANGES(docs): Document tmux < 3.2 support removal

why: Document breaking change for v0.49.x release
what:
- Add Breaking Changes section for 0.49.x
- Document removal of tmux 1.8 to 3.1c support
- Note removal of TMUX_SOFT_MIN_VERSION and deprecation system
- Point users to v0.48.x backport branch

Author: tony

CHANGES

@@ -32,7 +32,17 @@ $ uvx --from 'libtmux' --prerelease allow python
 
 <!-- To maintainers and contributors: Please add notes for the forthcoming version below -->
 
-_Future release notes will be placed here_
+### Breaking Changes
+
+#### tmux 1.8 to 3.1c support removed (#608)
+
+Support for tmux versions below 3.2a has been removed. This completes the
+deprecation announced in v0.48.0.
+
+- Minimum tmux version is now 3.2a (`TMUX_MIN_VERSION`)
+- Removed `TMUX_SOFT_MIN_VERSION` constant and deprecation warning system
+- Removed version guards throughout the codebase
+- For users on older tmux, use libtmux v0.48.x
 
 ## libtmux 0.48.0 (2025-11-28)