Is the IP PIN mandatory?

You are not required to use an IP PIN unless you have used one for past returns.   If you have not ever requested an IP PIN, you do not have to get one.   If you used one last year, you need one for 2025.

If you have been attributed an IP PIN by the IRS, then it's mandatory to enter it on your tax return, otherwise your return will be rejected when e-filed.

See this IRS document for more information.