As an example, Stripe went out of their way to get former employees paid.
If you are joining a startup as an employee and expecting your equity to worth something its important to be aware of the risks. And trying to sell as an ex-employee is a Risk.
Maybe I'm a cynic but having worked in employee equity I have seen more times that companies essentially turn their back on ex-employees than i have seen them actively helping them on liquidity transactions like tender offers.
I'm not convinced these events represent a significant enough number of shares to move the price, though. And regardless, many current and former employees will use this opportunity to dump more shares than just to cover withholding. I wouldn't be surprised if the number of shares sold normally at that time will dwarf the number sold to cover tax withholding.
For example, see how easy it is to get your bi weekly paycheck copies. Most will not reply at all.
It's super super gross, but it's unfortunately a thing. Hopefully that's not what's going on here with OP, but I wouldn't be surprised if it is. If the company is offering a sell-to-cover option to existing employees, they are certainly capable of doing so for former employees as well.
I'm watching this thread, but just as a reminder that it benefits the company to be as vague and complicated as possible for ex-employees trying to exercise their equity rights. You and your equity are effectively dead weight to the company now and it's in their best interest to get you to forfeit as much as possible. The best time to cash in your equity is always when you are still an employee.