When an installed plugin can't be loaded at startup as its .so file can't be found anymore, e.g. after switching form MySQL to MariaDB with a MySQL-only plugin installed, or when having a deprecated plugin still installed and the .so removed due to a version upgrade, the error log will contain warnings about not being able to load the plugin, but the plugin can't be removed with UNINSTALL PLUGIN or UNINSTALL SONAME.

It can only be removed by deleting the related row in the mysql.plugin table direrctly with DELETE FROM mysql.plugin WHERE name='...'

It should be able to remove a plugin with UNINSTALL even if it couldn't initialize for whatever reason, including missing .so file