Depending on whether or not the addon developer has provided an Uninstall script for the addon, an addon can be removed by executing the following command from the shell in the folder where your Tiki is installed, replacing "vendorname/packagename" with whatever your addon package is.
php console.php addon:remove vendorname/packagename
You will be shown a list of objects in your Tiki instance that will be removed if you choose to proceed with the removal. Note, there is NO UNDO, and if you are not sure, please keep a backup of your database before proceeding.
Once you are sure you want to proceed with the addon, as prompted, you can execute the actual removal as follows, , replacing "vendorname/packagename" with whatever your addon package is.
php console.php addon:remove vendorname/packagename --confirm
There might be left over items even after the removal command is executed that might have to be removed manually. Some documentation on what to look for should be provided by the addon developer.
Depending on how comprehensive the addon developer has included items for removal in the uninstall script provided, some items might remain. This depends entirely on what the addon developer has specified in the uninstall script.
While using the addon, a user might create content which might not be possible to be removed through the uninstall script as it may not be possible for the script to know what has been created subsequent to the installation of the addon. Nonetheless, sometimes it's possible, for example, even if users create a tracker item in a tracker that is removed by the uninstall script, the tracker items within that tracker will also be removed.
The information as to what is removed is stored in an "uninstall.json" file within the addon's folder. An addon developer might provide different variants of this file (from their website/linked from their documentation) that provide different levels of removal, which can be used instead of the default shipped one. For example, depending on your situation you might only want to remove some but not all the data that the addon or addon users have created.