What happens if the tax set as the default tax is deleted from the account?

If the tax that was configured as the default tax is deleted from the account, the setting automatically reverts to “Match Product Tax” and a warning message appears in the Default Tax section in settings: “The previously selected default tax no longer exists. The setting has been reset to Match Product Tax.”

You should then review the default tax setting and reconfigure it if you do not want to keep the default option.