"Fields pending deletion" stopping module unistall - how to delete manually?
It turns out that deleted fields in Drupal 8 are stored in the key_value table.

To find them:

select * from key_value where name = "field.field.deleted" or name = "field.storage.deleted"


The values are stored as a blob, but can be viewed (e.g. in phpmyadmin you can click on it to download, and view in vim or similar). The value does not make much sense to a human, but having gone through the code printing and error-logging various things it was clear to me that these entries were what was holding up the uninstall. I backed up the table and deleted the entries, and then was able to uninstall the module.

The same suggestion is made here in answer to a related issue:

https://drupal.stackexchange.com/a/230199/25792