Breaking changes in code
Plugin's success() and error() methods have been removed, use resolve() and reject() instead.
Some deprecated plugin definitions and methods such as registerWebPlugin have been removed, check the Capacitor 3 plugins upgrade guide for more information.
Capacitor's helper properties platform and isNative have been removed, use getPlatform() and isNativePlatform() methods instead.
Android
BridgeFragment class has been removed, if a plugin was using that class to present a Fragment, they'll have to create their own version of it.
iOS
Add SPM support
Capacitor 6 added experimental SPM support, you can add support for your plugin following Converting existing plugins to SPM
Updating Capacitor to 7.0 in your plugin
Using @capacitor/plugin-migration-v6-to-v7
From the plugin folder, run npx @capacitor/plugin-migration-v6-to-v7@latest and it will perform all the file changes automatically.