Is payment of interest tax included/excluded in National Income?

No,

The payment of interest tax will not be included in the national income as it is a compulsory transfer payment to the government.

Explanation:

Any tax to the government is a transfer payment as the government does not guarantee any productive services and goods in return.

Thus any economic transaction where no factor services against it, is a transfer payment.

It does not add to the flow of goods in the economy.

Thus, It is not included in the National Income.