Should I need to upgrade to a paid TurboTax filing plan because I have student loan interest, or is this covered in simple filing?

You can enter student loan interest using the Free Edition.  You do not have to upgrade for that.  

TurboTax Online: Important Details about Filing Simple Form 1040 Returns

If you have a simple Form 1040 return only (no forms or schedules except as needed to claim the Earned Income Tax Credit, Child Tax Credit or student loan interest), you can file for free yourself with TurboTax Free Edition, or you can file with TurboTax Live Assisted Basic at the listed price. Roughly 37% of taxpayers are eligible.

Examples of situations included in a simple Form 1040 return (assuming no added tax complexity):

• W-2 income
• Interest, dividends or original issue discounts (1099-INT/1099-DIV/1099-OID) that don’t require filing a Schedule B
• IRS standard deduction
• Earned Income Tax Credit (EITC)
• Child Tax Credit (CTC)
• Student loan interest deduction
• Taxable qualified retirement plan distributions
•  

Examples of situations not included in a simple Form 1040 return:

• Itemized deductions claimed on Schedule A, like charitable contributions, medical expenses, mortgage interest and state and local tax deductions
• Unemployment income reported on a 1099-G
• Business or 1099-NEC income (often reported by those who are self-employed, gig workers or freelancers)
• Stock sales (including crypto investments)
• Income from rental property or property sales
• Credits, deductions and income reported on other forms or schedules