Can an non-taxable income account be excluded from P&L?

Yes
Create a subtotal near the bottom of your COA on the profit and loss side called “Taxable income” then put non taxable income (and expenses) below that on the COA.

For one off special purpose payment it also more clearly illustrates actual business performance imo.

Where that COA layout is problematic, a more general solution is described in the following ideas threads Income tax payable and New Custom Fields and Chart of Account