Please see the following issues for breaking changes to plugins and themes.

https://github.com/NodeBB/NodeBB/issues?utf8=✓&q=is%3Aissue+milestone%3A1.9.0+label%3A"breaking+change+(themes)"+
https://github.com/NodeBB/NodeBB/issues?q=is%3Aissue+milestone%3A1.9.0+label%3A"breaking+change+(plugins)"

If you have plugins that are affected, please update your plugin and add the below to your package.json

"nbbpm": { "compatibility": "^1.9.0" }

to your package.json.