If a tax return is printed and mailed, an explanation for any reason can be attached to the paper tax return.
An e-filed tax return does not have option to have a free form section for writing explanations because the IRS does not support a free form option on electronically filed federal tax returns.
At the end of 2021 there were over 65 million recipients of Social Security benefits. The option to have federal taxes withheld from the benefits is up to each individual recipient. Millions of recipients do not have taxes withheld from their benefits (myself included). In no way does this cause a delay on the federal tax refund.
