LYFT and many recently IPO'd tech companies have been doing monthly vesting, with no cliff. So basically with all their massive hiring, employees have already been dumping shares.

The conditions on the unvested portions typically are destroyed. With options are available to purchase for exercise. That's pretty exclusive to startup life, but the same concept in RSUs at big tech where only the vested portion is sellable and the rest is destroyed.

That being said, a contract can say anything.

The other thing I would add is that new employees usually get some cash value for their level divided by the current stock price. Many laid off employees were hired when the stock price was higher. If they have to replace these employees with new ones in a timeframe before the stock price has recovered then they suffer more dilution.