Not sure about this. A random thing[0] I found says:
"RSUs are considered supplemental income, and as such, the income you receive from them is subject to withholding taxes. The IRS requires a federal withholding rate of 22% for supplemental income up to $1 million, and 37% for income exceeding that amount."
(It also links to the relevant IRS publication, but I am not interested in poring over it right now.)
So I believe the employer is required to do that withholding.
It does seem pretty sketchy that current employees have access to a sell-to-cover option, but former employees do not. This feels like the company trying to screw over former employees, since they don't care if a former employee gets mad at them. Over the years I've heard of quite a few companies (and founders) who have this bizarre, messed-up belief that employees who leave the company pre-IPO shouldn't be entitled even to their vested equity.
I glanced through an old RSU plan doc from a former employer, and it does talk about withholding, including a sell-to-cover option, but the wording sounds (to me, anyway; IANAL) like they aren't required to offer that option, only that they may do so. Obviously I have no idea what OP's legal docs say (or if I'm even interpreting this doc I do have correctly); a lawyer would need to go over them.
[0] https://www.harnesswealth.com/articles/what-you-need-to-know...