Notice 2021-20 doesn't contain any information on tips as wages. However, N-2021-49 does.

The Treasury Department and the IRS have been asked whether tips are qualified wages. In general, qualified wages are limited to wages as defined in section 3121(a) of the Code and compensation as defined in section 3231(e) of the Code, with certain modifications relating to the inclusion of qualified health plan expenses and, for calendar quarters in 2021...

Section 3121(a)(12) of the Code excludes from the definition of “wages” tips paid in any medium other than cash and cash tips received by an employee in any calendar month in the course of employment by an employer unless the amount of the cash tips is $20 or more. Accordingly, if cash tips received by an employee in a calendar month amount to $20 or more, all of the cash tips received by the employee in that calendar month are included in wages. Similarly, section 3231(e)(3) of the Code provides that the term “compensation” includes cash tips received by an employee in any calendar month in the course of employment by an employer unless the amount of such cash tips is less than $20. Under section 3121(q), tips received by an employee in the course of the employee’s employment are considered remuneration for that employment (i.e., wages) and are deemed to have been paid by the employer for purposes of the taxes imposed by section 3111(a) and (b) of the Code. Thus, for purposes of chapters 21 and 22 of the Code, cash tips of $20 or more in a month are treated as wages paid by the employer.

Therefore, any cash tips treated as wages within the definition of section 3121(a) of the Code or compensation within the definition of section 3231(e)(3) of the Code are treated as qualified wages if all other requirements to treat the amounts as qualified wages are satisfied.

The Treasury Department and the IRS have also been asked whether an eligible employer may claim both the employee retention credit and the credit available under section 45B of the Code on the same wages. Section 45B(a) provides that, for purposes 23 of the general business credit under section 38 of the Code, the credit for employer social security and Medicare taxes paid on certain employee tips is an amount equal to the “excess employer social security tax” paid or incurred by the employer. The term “excess employer social security tax” means any tax paid by an employer under section 3111 of the Code (both social security tax and Medicare tax) on its employees' tip income without regard to whether the employees reported the tips to the employer pursuant to section 6053(a) of the Code. Consequently, the section 45B credit is available with respect to unreported tips in an amount equal to the “excess employer social security tax” paid or incurred by the employer. No credit, however, is allowed to the extent tips are used to meet the federal minimum wage rate. For purposes of this limitation, the federal minimum wage rate is the rate that was in effect on January 1, 2007. The credit is available with respect to Federal Insurance Contributions Act (FICA) taxes paid on tips received from customers in connection with the providing, delivering, or serving of food or beverages for consumption, if it is customary for customers to tip the employees. (pp. 21-23)