Stripe was already worth $9B in 2016. If you joined then, it could have been prohibitively expensive to do an 83b election. The whole point of RSUs is that you don't owe anything until there is a liquidity event, unlike options which you may be required to exercise or lose while the company is still private.
However, RSUs only get this favorable treated (i.e. you've been given something of value, but can defer paying taxes on it) because they technically expire worthless if the company does not have a liquidity event in time. Thus far no successful tech company (that I know of) has screwed over its employees by casually choosing not to have a liquidity event and letting years worth of RSUs grants all expire worthless.
Stripe is trying to arrange liquidity for its employees who were granted RSUs in e.g. 2016, and that expire in 2023. Those employees have not had to pay taxes as the RSUs vested, but will have a large tax bill if those RSUs do anything other than expire worthless...