Context for folks who haven't encountered vesting before: vesting is an arrangement where, instead of getting all of your ownership up front, you earn it over time. For example, you might have a fairly common arrangement in Silicon Valley where you get 1/4 of your equity after working for a year and the remaining 3/4 over the next 3 years. This pre-commits to what happens regarding ownership if someone leaves 18 months (or 18 days) into running the company.

Atlas C corporations with multiple founders have a vesting schedule built-in (4 year vesting; 1 year cliff), the market standard in Silicon Valley.

Vesting in an LLC is complicated:

Because an LLC is a partnership, there can be complicated consequences of someone stepping back from an operational role (like e.g. being a founder) but remaining an owner of the business. We didn't think it was appropriate to have a one-size-fits-all answer for this: some companies might choose to buy out the departing founder, some might choose to pay them dividends on an ongoing fashion, and some might not have financial bandwidth to pay dividends but might want them to participate in upside in the event of a future acquisition.

Conversely, adopting vesting establishes some fairly substantial tax consequences (83(b) elections, etc).

This is heavily dependent on facts-and-circumstances.

Vesting is more of an established concept, with off-the-shelf support throughout the ecosystem, for C corporations. For those who want it in an LLC, please speak to an attourney. You can still adopt it for your Stripe Atlas LLC, it just isn't built in to the default experience.