The way I read it I believe it is mandatory, as it states the employer is to withhold the taxes that weren't withheld in first quarter 2021. It also says the employer can take other steps to collect from employee, and that probably means the employee isn't working  there any longer. Just an opinion, but there is a phone number to call in the notice if it effects you. I also think if the employer did not comply, some employees would be upset.