How to hook into unregistering a widget instance?

Yes, this can be done. Ultimately, adding and removing a widget to/from a sidebar means updating an option in the database with a call like this:

update_option( 'sidebars_widgets', array( ... ) )

If you look at the code of update_option you see that there is a filter called just before the actual update. So if you want to do something before a widget is removed you can hook into:

add_filter ('pre_update_option_sidebars_widgets','wpse71075_pre_remove_widget',10,3);

At this point you only know that the list of active widgets is going to change, not whether it is addition or a removal. However, you do have $value and $old_value of the option, which you can compare inside your filter function (an array_search for a widget’s label might be enough).

Read the answer to this question to learn about the structure of the sidebars_widgets option. It depends on your goal what you actually want to detect and do.

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