For normal WordPress operations, such as posting blog posts, uploading media files, posting comments, creating new WordPress users and installing WordPress plugins, the MySQL database user only needs data read and data write privileges to the MySQL database; SELECT, INSERT, UPDATE and DELETE.
Therefore any other database structure and administration privileges, such as DROP, ALTER and GRANT can be revoked. By revoking such privileges you are also improving the containment policies.
Note: Some plugins, themes and major WordPress updates might require to make database structural changes, such as add new tables or change the schema. In such case, before installing the plugin or updating a software temporarily allow the database user the required privileges.
Digging deeper, We found that in order to operate fully (automated updates, plug-in installation/uninstallation, etc.), WordPress requires some additional permissions:
DELETE ALTER (for updates) CREATE TABLE DROP TABLE
Also, not referenced but it makes sense:
INDEX
If you want to limit your DB use, start with these 8 privileges and test fully to make sure things work as expected.