Manual:$wgContentHandlers/en

Details
Plugins for page content model handling. Each entry in the array maps a model ID constant to a PHP class name.

When undeploying an extension that provides handling for a content model, that content model becomes unsupported, leading to MWUnknownContentModelExceptions to be raised. To avoid this, the content model in question can be defined to use the :

This allows pages that use the new unsupported content model to still be accessible, even though their content can no longer be shown.

Alternatively, if the content model in question is a text based model, it can be re-defined to be handled as plain text:

The content of pages that use this content model remain readable and editable, but lose any special capabilities and markup support.