They are double-trigger (source: I am a Stripe RSU holder). However, they also have an expiration date. The expiration provides the "substantial risk of forfeit

They are double-trigger (source: I am a Stripe RSU holder). However, they also have an expiration date. The expiration provides the "substantial risk of forfeiture" that the IRS requires in exchange for not taxing the RSUs at the time you receive them.