Display custom taxonomy as dropdown list

Below is the easiest solution I use for displaying custom taxonomy as the dropdown instead of checkboxes.

1/ Use third party WordPress plugin ‘ACF’ to create the taxonomy ‘Relational’ field type and display it as dropdown as shown in below screenshot.

https://wordpress.org/plugins/advanced-custom-fields/

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For displaying taxonomy selected you can refer documentation of ACF for relational taxonomy field type.

https://www.advancedcustomfields.com/resources/taxonomy/

2/ Hide taxonomy checkbox by simply adding ‘meta_box_cb’ as ‘false’ when registering your custom taxonomy.

Hope this helps..!!